The correct option is B sweet
A subject complement is an adjective, a noun or a phrase that follows a
linking verb and gives more information about the subject.
In the given sentence, "sweet" is an adjective that complements the noun in the subject "soup", hence option C is correct. "The soup" is the subject itself and "tastes" is the verb. Therefore they cannot function as subject complements and options A and B are incorrect. Option D is incorrect because the correct option lies in C.