Answer:
The sentence above is a complex sentence.
Step-by-step explanation:
A complex sentence is a sentence that contains an independent clause and one or more dependent clauses. An independent clause can stand alone as a sentence (2), but dependent clauses can't. Dependent clauses do not represent a complete idea, so they depend on the main clause to complete their meaning (3). In the sentence above, there is one independent clause and one dependent clause.
1) Holly sang along as the band played her favorite song
2) Holly sang along
3) *As the band played her favorite song
(3) is an adverb clause. Adverb clauses are dependent clauses that function as adverbs. In this case, the clause adds information as regards the time in which the singing took place or even tells how it happened (as the band played her favorite song).