The correct option is C so pretty
A subject complement is an adjective, a noun or any phrase that follows a
linking verb and gives more information about the subject.
In the given sentence, "pretty" is an adjective of quality that follows the linking verb "looks" and gives more information about the subject "The cake". Hence it is the subject complement. Based on this, option C is correct and A and B are incorrect. Option D is incorrect because the correct answer lies in option C.