A gerund is a verb ending with -ing, so it’s not A.
A participle would be sung, so it’s not B either.
An infinitive is a whole verb (+to), like ‘to sing’ in this sentence..so this one is right.
An appositive is a noun that follows another noun (in commas) and which describes that noun.
(For Example: Ralph, my father, built that dollhouse. )
So, it’s C.