Which pronoun best completes the sentence?
each dancer gave the performance all.

Well, it's asking for pronouns. 

your options are: Me, you, him, her, it, us, them, our, ours, your, yours, his, hers, its, theirs, their

you are talking about dancers here, so you can get rid of all the first-person and second-person ones, as well as the plural ones, because the subject is "dancer."
This leaves you with: his, it, and her. "It" doesn't make sense at all. My first instinct as a dancer myself is to say "her", since most dancers are female. However, the sentence doesn't specify gender, and even one male in a whole class of female would make this "his". And, who knows? It could be a guy dance class. therefore, the safe answer is "his", although your teacher will likely take "her" as well for an answer

The answer is d. prior knowledge. she breaks it into parts in order to figure out what other words have the same parts to the word and put the meanings together.