Students are highlighting words that begin with "un-" and then identifying a word that comes after "un-" and discussing how the meaning of the original word changes when "un-" precedes it. In this case, students are learning about how to determine word meaning primarily through their understanding of which of the following?

While background knowledge plays a role in comprehension, it is not the primary focus when identifying how "un-" modifies a word's meaning. Students may have prior knowledge of the root word, but without understanding the specific function of the prefix, they cannot accurately determine the changes in meaning.

Context is important for understanding how a word is used in a sentence, but it does not directly explain how the meaning changes when the prefix "un-" is applied. Contextual clues might help students infer meaning, but they rely on knowledge of word parts to grasp the specific transformation that occurs with prefixes.

Synonyms are words with similar meanings, but they do not aid in understanding how prefixes change word meanings. Recognizing synonyms may help students find alternatives or similar words, but it does not provide insight into the structural changes that prefixes like "un-" impose on root words.