In the word “accelerate,” the “celer” portion means quick, coming from the Latin "celeriter," meaning quickly. The word “celerity,” therefore means quickness. It would be used in sentence like, “I am often surprised by the celerity of the deadly tree squirrel, which can kill its prey more quickly than many other animals.”

The word “amble” comes from the Latin for “to walk,” which is found in English words like “ambulatory” and “ambulate.” It means “to walk in a relaxed manner.” Among the options, “saunter” best describes such walking. One might say, “She ambled about the county paths, not wishing to hurry past any of the beauties of the autumn scenery.”

When something “mollifies,” it makes something softer. An “emollient” is a substance (e.g. a cream) that helps to make the skin softer. Although the word “mollify” can be used in this general sense of “softening,” it is often used to describe the specific case of “softening” emotions, that is to make someone less anxious. For this reason, “placate” is the best option among those given.

The word “vociferous” is quite recognizably related to the words “voice,” “vocal,” and so forth. The “-fer” portion of the word is derived from the Latin for “to carry,” as is found in the words “transfer” (to carry across) and “aquifer” (something that carries water between two or more places). The word vociferous means “carrying voice” in the sense of being loud or outspoken. For example, one could say, “At the proposition that she should change her political affiliation, the young woman became particularly vociferous, arguing passionately for her beliefs.”

Someone who is "diligent" pays attention to the details of something. For instance, a diligent student is one who studies much and does his or her work in a thorough manner. The word comes from the Latin for to love or take delight in, which is found in the word “predilection,” meaning a preference for something. If someone is "diligent," that person probably has a love for the activity on which he or she is focusing. A "meticulous" person is very careful regarding details, so this is the best option for this word.

The word “verbose” comes from the Latin for “word.” It is related to words like “verbal” and “verb.” It means using too many words to communicate something. Although the word “wordy” is a bit informal, it means just this—that someone uses too many words.

The word “onerous” comes from the Latin for burden, so something that is “onerous” is something that is burdensome. Often it means something requiring much effort. It would be used in a sentence like, “The sixteen-hour work days in the mines were so onerous for the poor miners that they were continuously exhausted from the difficult work.” The word is related to “exonerate,” meaning to set free from blame, or, more literally, to lift the burden of blame from someone.

The word “vacuity” is related to the words “vacuum,” meaning empty space, and “vacate,” meaning, “to empty out an area.” “Vacuity” can be used to describe a mere empty physical space, though it usually is used to indicate intellectual emptiness. For instance, one could say, “After years of listening to the vacuity of his classmates’ opinions, the student was glad to graduate and be freed from their unthinking opinions.”

The word “vibrant” is related to words like “vibrate” and “vibrato.” In a sense, something that is “vibrant” is “shaking with energy.” Often, the word is used to describe something that is dynamic, lively, or energetic. It can mean more than physical energy, for one can speak of a “stimulating, vibrant work environment” or a “vibrant imagination.” The word can also mean “colorful.”

The word “simultaneous” comes from the Latin root “simul,” meaning “together.” Though in some ways related to the word “similar,” it indicates this notion of two things being “together” or “at the same time.” The word “concurrent” literally means “current with” or “happening / existing at the same time as something else.” The “con-” prefix is the same as “cum” found in “cumulative” and “com-” as found in “community.” It means “with.”