Answer:
because he called off the party
Step-by-step explanation:
An adverbial clause is a dependent clause which functions as an adverb.
The dependent clause "because he called off the party" functions as an adverb of reason. It offers a reason for the main clause "He hasn’t shown up here". It is answering the question "Why?"
Such clauses usually begin with subordinating conjunctions because, as, since. As we see, this particular clause starts with because.