Answer:
he; mob boss
Step-by-step explanation:
This is a complex sentence composed of a dependent clause linked to an independent clause. This dependent clause is a relative clause, particularly a that-clause, but in this case, the pronoun "that" is omitted. There are two subjects, he, that belongs to the independent clause and the mob boss, that belongs to the dependent clause.
He didn't know (that) the mob boss paid off the cop.