A gerund is a verb ending with -ing, so it is not A.
A participle can be sung, so it is not B both.
An infinitive is an entire verb (+to), like ‘to sing’ on this sentence..so this one is true.
An appositive is a noun that follows one other noun (in commas) and which describes that noun.
(For Instance: Ralph, my father, constructed that dollhouse. )
So, it is C.