How might a reader find captions useful? readers can use them to identify main ideas at a glance. readers can use them to make connections to knowledge. readers can use them to obtain summary information quickly. readers can use them to understand a visual element's content.

D.  Readers can use them to understand a visual element's content.

Sometimes when you are researching or reading (be it a book, magazine, online article, etc.), you will encounter pictures, drawings, charts, and graphs.  Beneath these images, there will sometimes be a caption that includes information which will elaborate or describe the content in order for you to gain a better understanding of what it is you are seeing or for the author detailing some specific information he or she feels you should know in order for you to better understand a topic/point being presented or argued.

D

Explanation:

there purpose is to entertain

what excerpt? i would generally think a.