Final answer:
The sentence fragment is "Legs for hopping and lungs for breathing air" because it lacks a complete thought and an independent clause, missing either a subject or a verb.
Step-by-step explanation:
The sentence fragment among the provided options is "Legs for hopping and lungs for breathing air." This group of words is a fragment because it lacks a complete thought and does not have an independent clause: it does not express a complete idea and is missing either a subject or a verb that can stand on its own as a sentence. A complete sentence in comparison would provide additional context, such as "Froglets develop legs for hopping and lungs for breathing air during metamorphosis." The other sentences provided are complete, as they contain both a subject and a verb, forming a complete thought.