Answer 4 cannot be it right off the bat, since compound-compound doesn't exist.
If a compound - complex sentence loses the dependent clause, it is no longer complex, since complex sentences require a dependent and to independent clauses conjoined with a comma and conjunction such as "and". So answer 3 can not be it.
So that leaves either simple or compound.
The sentence is still contains an independent clause from when it was compound-complex, therefore answer 2, compound, is the answer.