The correct option is C crying
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, "crying" is a gerund which acts as a subject complement to the noun in the subject "baby", hence option C is correct. "The baby" is the subject and "is" is the linking verb, they cannot act as subject complements. Hence options A and B are incorrect. Option D is incorrect because the correct option lies in C.