B: I might decide that I didn't really hit you very hard.

This is right, D is the correct answer though, so be sure to read all of the options before making your final choice on the test.

According to balance theory, we strive to have balanced cognitions.  It is impossible to change the behavior of striking another, so instead a cognition is changed.  Deciding that I did not hit you very hard, would allow me to reduce my dissonance about having hurt you and I could still like you, see below