There are multiple classmates described in the sentence. Therefore, there should also be multiple (or plural) singers. However, the word here is the singular, “singer,” which is incorrect.

In the clause "the act of pairing socks is by far the most tedious tasks," "act" and "tasks" are referring to the same thing, so "task" must be singular like "act" is. "Tasks" is plural and is therefore incorrect.

The sentence uses the phrase "seven toes on each feet," and this is incorrect because "each" is used to describe a singular noun, but "feet" is plural. Changing "feet" to "foot" corrects this error.

This sentence refers to two prizes, first and second, for two people, Bob and Frank. The plural noun "prizes" should be used. The corrected sentence reads, "Bob and Frank won the prizes for first and second place in the elementary school baking contest."

This sentence has a discrepancy in number. Lewis and Clark are two different people, so they are two different frontiersmen. The corrected sentence reads: "Lewis and Clark, the now legendary explorers of the Louisiana Purchase, were chosen for their scientific knowledge and skills as frontiersmen."

In this sentence, there is a discrepancy in number. There are three pumpkins, so there should be multiple winners. The corrected sentence reads: "All three of the pumpkins that Gerald entered into the contest became top-ten winners; needless to say, he was thrilled."

"Students" is plural, so the plural word "fans" should be used in the sentence. However, the word used in the sentence as it is written is the singular “fan,” which is incorrect. 

This sentence's error is a discrepancy in number. Two children are mentioned, "Jordan and Marvin," so logically, they would grow up to become two different lawyers. The sentence uses the singular noun "a lawyer" where it should use the plural noun "lawyers," so "lawyer" is the correct answer.

The corrected sentence reads, "Jordan and Marvin’s mother has decided that they will become lawyers when they grow up, but they seem more artistically inclined."

This sentence's error is a problem with agreement of plurals. Multiple "caterpillars" should form multiple cocoons, but the sentence uses the singular noun "a cocoon," which doesn't make sense. "A cocoon" is thus the correct answer, and the corrected sentence reads, "As part of their development, caterpillars form cocoons and eventually emerge as moths or butterflies."

As with pronouns, when a noun or group of nouns is replaced in a sentence by another noun, they must agree in number. The multiple subjects "Dedication," "work," and "attention" call for a plural word.  A simple correction would be "important factors"