When you are reading a text and find an unknown word, one way you can infer the meaning of that word is by using context clues, that is to say, by looking at the surrounding information of that unknown word. There are many types of context clues, A: Direct explanation and C: Synonym are two of them.
Sometimes when the author of the piece of writing is aware that the word he or she is using is not common to most readers, they may tend to provide a definition, an explanation or an example that helps us understand the word. Other times, there is not a direct explanation available but we can infer the meaning of the word by identifying the synonyms that appear surrounding the word.