What is the difference between a flat character and a round character?

A) a flat character takes up fewer lines and pages

B) a round character is more complex

C) round character are harder to find in good fiction

D) flat characters are preferred by most readers and writers.

There isn't that big of a difference between flat and round characters, however both are needed for the story to go on. Flat characters don't show as much emotion and the reader knows a lot about a round character's personality. Flat characters are usually side characters while round characters are generally main characters.