The sentence "As immigration patterns changed in the mid-1800s, most immigrants came from Germany and Ireland, which are countries in western Europe." that you have provided for us would be known as a complex sentence. How?
Since this sentence involves commas, it would not be a "simple sentence".
It's also not a compound-complex sentence because you have to have two independent clauses that can stand on their own and make sense.
We can cross out compound sentence since this sentence has two dependent clauses, but only one independent clause and compound sentences needs to have two independent clauses.
Therefore, the correct option out of the other answer choices listed above would be: A.
