A simple sentence is best described by the option: "A sentence that contains a subject and expresses a complete thought."
This option highlights the essential characteristics of a simple sentence, which includes having a subject and a verb and conveying a complete idea.
However, the option as given includes a typo with "anniversary," which should be "and," but the overall idea is correct.
If there is a need for a more formal definition, a simple sentence contains one independent clause and no dependent clauses.