Answer:
I had to prove my identity: to gain entrance to the building I was required to fill out an impressive number of forms.
Step-by-step explanation:
Compound sentences consist of two independent clauses, which can stand on their own and cannot be joined by a comma. Instead, it is proper to combine independent clauses with a semicolon or colon. In the chosen sentence, the independent clauses are linked with a colon. Thus, the second clause introduces an explanation to illustrate the first clause.