Final answer:
The sentence in question is a simple sentence because it contains one independent clause and no subordinate clauses.
Step-by-step explanation:
The sentence 'She was wearing a large gold necklace set with jewels' is a simple sentence. A simple sentence has just one independent clause and no subordinate clauses. This sentence has one subject, 'she,' and one predicate, 'was wearing a large gold necklace set with jewels.' It does not contain multiple independent clauses (which would make it compound) nor does it have a dependent clause (which would make it complex or compound-complex).