Use “which” for added information after a comma. “That” is for cases like this, where the information is an important part of the sentence and there is no comma. The corrected sentence reads, "They were very excited because they had baked their friend a triple layer chocolate cake that they knew he was going to love."

“Which” is used to signal modification to a phrase when the modifying dependent clause comes after a comma. “That” is used to signal modification to a phrase in which there is not a comma after the subject being modified. Here, the subject being modified ("one" in reference to the cupcake) does not have a comma after it. Therefore, “that” is appropriate, not “which.”

This sentence confuses the words “that” and “which.” "That" is used to begin restrictive clauses—clauses that provide necessary information about the sentence's subject and are not set apart from the rest of the sentence by commas. (An example of this would be, "The kitten that had a spot on its nose was Marlene's favorite.") "Which" is used to begin nonrestrictive clauses—clauses that are introduced by commas and provide extra information that could be omitted from the sentence without significantly altering its meaning. (An example would be "The kitten, which had a spot on its nose, slept peacefully.")

In the given sentence, the information "that of course was the focus of the exam" follows a comma, signaling that it is a nonrestrictive clause and should begin with "which," not "that." You could correct the sentence by changing "that" to "which," making it, "I knew all of the information except for the material on the Civil War, which of course was the focus of the exam."

This sentence uses the word "which" where it should use the word "that," so "which" contains the sentence's error and is the correct answer.

"That" is used to signal the start of a restrictive clause, a type of clause that provides necessary information about the sentence's subject and is not set apart from the rest of the sentence by commas. (Example: "The fish that was frying in the pan smelled delicious.") "Which" is used to begin nonrestrictive clauses, which are introduced by commas and provide extraneous information that could be omitted from the sentence without significantly altering its meaning. (Example: "The pancakes, which were warm, were Jenna's favorite part of the breakfast buffet.")

In the given sentence, the phrase "which traveled with us throughout Europe, posing in front of historic landmarks" tells us important information that helps define its referent, so "that" should be used, not "which," making the corrected sentence, "I never admitted it to my daughter, but I lost the little sock puppet that traveled with us throughout Europe, posing in front of historic landmarks."

This sentence uses the wrong form of its/it’s. “Its” is a possessive pronoun, while “it’s” is a contraction of “it is.” The latter is what is appropriate here. The corrected sentence reads, "My grandmother tells me that it’s the best time of the year to make homemade apple sauce, but she still won’t pass along her secret recipe."

This is an example of commonly confused words: in this case we need the possessive pronoun "their."

"They're" is a contraction for "They are" and "there" indicates a location.

In this sentence, the incorrect spelling of their/there/they’re is used. “Their” is a possessive pronoun (e.g. their giraffes), while “there” is used for existence (e.g. there are naked mole rats). The corrected sentence reads: While I expected giraffes and monkeys at the zoo, I had no idea there were going to be naked mole rats as well.

In this sentence, the wrong version of you’re/your is used. “You’re” is a contraction of “you” and “are,” while “your” is the possessive pronoun that we want in this sentence. The corrected sentence reads, "I’ll admit that I was a little skeptical about the idea of chickens as pets, but you’re chicken Spike’s friendliness and character took away all of my doubts."

"Affective" is easily confused with "effective." "Affective" means to influence someone emotively, while "effective" means to produce a desired result. 

Here, the problem is that the regulations will not be "effective," or produce the desired result. However, the sentence uses the word "affective," which is not the proper word choice. Therefore, "affective" is the part of the sentence that contains an error.

"Its" and "it's" are a pair of homonyms that are often confused. Without the apostrophe, "its" is the possessive form of the pronoun "it." On the other hand, "it's" is the contraction of "it" and "is." In this sentence, "it is no surprise" captures the sentence's meaning, but the possessive form of the pronoun "it" does not make any sense. So, "its" is the part of the sentence that contains the error.