Answer:
Mighty
Step-by-step explanation:
Mighty is the most applicable answer to replace "powerful". To clarify, let's go over each option.
Famous: The word "powerful" is not a synonym for "famous", especially in this context. I.e, the sentence does not describe the animal's legs as well-known, rather, it describes them as strong.
Inept: The word "inept" means unable to complete a certain task. The sentence is describing the animal's legs as strong and mighty, which is the opposite of inept.
Notable: The animal's legs may be notable, or worthy of notice, but that is not how the sentence describes them.