To make the sentence "I bought popcorn ____________." a complex sentence, you can add a dependent clause. The option that fits best is:
before the game started
This forms a complex sentence because it adds a clause that cannot stand alone (it depends on the main clause), giving more context to when the action took place.
Other options like "with butter and salt," "today," and "and a drink" do not create complex sentences because they do not include a dependent clause. They may enhance the sentence but do not add complexity.