Ed M. answered 03/24/16
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Help with grammar, French, SAT Writing, the TOEFL and ESL.
I would think your choice "a) contrast" "BEST [sic] describes" the task of an essay whose topic is "How is capitalism different from socialism?" since the implied assertion of that topic is of course that these two economic systems are different and your specified purpose is to explain "how" they are so, i.e., you must contrast them.
In contrast [ain't I clever], none of the verbs of your other choices, even compare of choice "b)," necessarily includes the idea of highlighting differences, or at least that is not their principal meaning as it is for contrast.