Answer
A.
Work Step by Step
Homologous chromosomes, two different versions of the same chromosome inherited from parents, exchange genetic material because of crossing over, formation of chiasmata (A.). Sister chromatids do not exchange (B.) since they are identical and the exchange would produce no new combinations, and C. is wrong for the same reason (B. and C. say the same thing in different ways). D. is wrong because all chromosome pairs undergo crossing over.