Answer: C) when the surrounding words are unfamiliar.
Explanation: context clues are hints that an author gives to help define a difficult or unusual word within a book. The clue may appear within the same sentence as the word to which it refers or it may follow in the next sentence. They can be synonyms, definitions, explanations, etc. When we found a word or phrase that we don't understand, and all the surrounding words are also unfamiliar, we should refer to a dictionary to be able to understand the text.