Which of the following is a complete sentence?

• A. Running fast.
• B. She ran fast to catch the bus.
• C. Fast running.
• D. To catch the bus.

Answer: (B)

A complete sentence contains a subject and a predicate and expresses a complete thought. The option that fits this definition is "She ran fast to catch the bus." This sentence clearly identifies the subject, "She," and the action, "ran fast," providing a complete thought about what she did and the purpose of her action, which is to catch the bus.

In contrast, the other choices lack essential components that make them complete sentences. "Running fast" is a phrase that does not have a subject performing the action. "Fast running" similarly lacks a subject-verb pairing and is just a description. "To catch the bus" is an infinitive phrase that indicates purpose but does not stand on its own as a complete thought.