Answer:
The complex sentences are indeed:
You will need to talk to Mr. Cortez if you want to try out for the play.
Although many people dislike reptiles, I think snakes are fascinating.
Once I finish knitting this scarf, I will make a matching hat.
Step-by-step explanation:
A complex sentence is formed by joining an independent clause and a dependent one. An independent clause can express a full thought on its own. On the other hand, a dependent clause needs the independent one to give it context, so that it makes sense. It does not express a complete thought on its own. The independent and dependent clauses are joined by subordinating conjunctions. The subordinating conjunction "if" expresses a condition; "although" expresses contrast; and "once" expresses a transition of time. They introduce the dependent clause.
With the explanation above, we can conclude that the correct options are:
You will need to talk to Mr. Cortez if you want to try out for the play.
Although many people dislike reptiles, I think snakes are fascinating.
Once I finish knitting this scarf, I will make a matching hat.