Identify the incomplete sentence.

The phrase "While they were sleeping" is considered an incomplete sentence because it is a dependent clause. Dependent clauses do not express a complete thought and often leave the reader wanting more information. This particular clause introduces a condition or action but does not provide the main idea or action that would make it a complete sentence. To become complete, it would need an independent clause to finish the thought, such as "While they were sleeping, the dog barked."

In contrast, the other options provided (everything worked out all right, he wept, and babies are sure cute and cuddly) are all independent clauses that stand alone as complete sentences. They each contain a subject and a verb and convey a full thought, which is why they are complete sentences.