D. secret; furtive
Synonyms are words that have the same basic meaning. It should be noted that synonyms will not always have the exact meaning of each other, which is why they are not always seamlessly interchangeable. For example, a secret is something that is typically information kept from one or more persons. Furtive means something done secretively. Thus, while both words basically mean the same thing (which is the definition of a synonym), they do not have identical meaning.
