The correct answer is: After we found the theater.
A Subordinate idea or Subordinate phrase serves as a source of additional information for a sentece - either as an opening, transition, or closing statement. A Subordinate idea must start with a subordinate conjunction, and must contain a subject and a verb.
Regarding the example, the correct answer contains After as a subordinate conjunction, we as a subject, and found as a verb. This phrase also serves as an opening or introduction to the rest of the sentence.
