Which is a true statement about the body of an essay?
a. the body develops the main idea of an essay.
b. the body appears in the conclusion of an essay.
c. the body states the main i dea of the essay.
d. the body sums up the main idea on an essay.

The answer is a. The body develops the main idea of an essay. 

Here's why the other answers don't work out:
b. The body appears in the conclusion of the essay--This is not true because the body of an essay, also known as the supporting paragraphs, fall between the introduction and the conclusion, not in the conclusion. (It comes before the conclusion.)
c. The body states the main idea of the essay--The introduction should state the main idea of an essay, not the body.
d. The body sums up the main idea of an essay--The conclusion sums up the main idea of an essay, not the body.

Hope this helps!