Read the sentence.
we study mythology.
what should the writer add to the end of the sentence in order to create a complex sentence?
1. in the morning before english and science.
2. but we also study math and geography.
3. if we have free time in the afternoon.
4. and we learn the history of ancient greece.

3.if we have free time in the afternoon.  

In order for a sentence to be considered complex, it should have two things—an independent clause and a subordinate clause.  Because the sentence "We study mythology" is an independent clause, we are looking for a subordinate clause.  That said, response 1 is a prepositional phrase.  Responses 2 and 4 are independent clauses.  Response 3 is a subordinate clause.  One of the ways to tell if a clause is a subordinate clause is if the clause begins with a subordinating conjunction.  Out of all four possible responses to this question, only Response 3 begins with a subordinating conjunction—“if.”  Response 1 begins with a preposition, and Responses 2 and 4 begin with coordinating conjunctions.  

if we have free time in the afternoon.

Explanation:

answer: do you have any options

explanation: