The purpose of a caption is to

a. provide a visual to support text.
b. describe an image in a text.
c. organize and sort information.
d. divide a book into logical units.

The correct answer is: B) Describe an image in a text.

Sometimes when you are reading for a school assigment some images appear in the text, they usually come with a short text below it. That text describes the image or explains what is going on in it. Without this short text, sometimes the images would seem senseless. Imagine you are reading about the chinese government and an image of some people appears, without the caption you could try to guess who they are or what they do, but with it, the information will be precise, in that way your lecture is going to be enriched.  

i think they are preparing for the battle of gaul. : )

i hope this you! : )

i’m alive so yes and the author is a technique idiom