Question

What type is the following sentence: “The stranger knocked on the door, and I swallowed hard as I opened it.”
simple
compound
complex
compound-complex

Answer 1

compund-complex.

Answer 2

The stranger knocked on the door - independent
I swallowed hard - independent (" and " is a coordinating conjunction)
as I opened it - dependent (" as " is a subordinating conjunction that joins an independent clause with a dependent clause)

This sentence contains 2 independent clauses and a dependent clause making it a compound-complex sentence.
Just like ucreategames said, and I agree :)