The correct answer is D. Example clue
Step-by-step explanation:
In texts, a context clue refers to word or words that act as the context of a complex or unfamiliar word providing information about the meaning of it. These context clues are of different types depending on whether they provide a synonym, an antonym, an example or a definition to define the unfamiliar word.
In the case of sentence "Places such as Stonehenge and Area 51 are enigmatic" the meaning of the word "enigmatic" that means "mysterious" is mainly suggested through two examples that are introduced by the transition word "such as" and include the "Stonehenge and Area 51" that are two places that depict the features of an "enigmatic place". Therefore, the type of context clue that helps the reader, in this case, is an example clue.