A complex sentence is the one which contains one independent clause and a dependent clause. The only such example here is C, where 'because the movie was already sold out' is subordinate, and 'they decided to go bowling' is independent.
To provide the details and arguments that support the main idea
Would be the body
To restate the main idea by summarizing the supporting details
Would be somthing like a conclusion
To grab the attention of listeners and introduce them to the main idea
Is an obvious introduction to the main part