Question

United we stand; divided we fall.
a. Pun
b. Idiom
c. Onomatopoeia
d. Antithesis

Answer 1

This is an antithesis because the phrase are opposites of each other. An idiom is using symbolic language to represent something. A pun is a play on words.  An onomatopoeia is giving inanimate objects human-like qualities.

Answer 2

I believe this to be D. Antithesis.