Step 1: Link tissue to protein:
Hard outer coverings of the body, such as nails, hair, horns and hooves, share one protein: keratin.
Step 2: Check the others:
Legumelin is a plant protein from legumes. Myosin is in muscle fibres and drives contraction. Insulin is a hormone that regulates glucose.
Step 3: Conclusion:
Only keratin belongs in nails.
Step 4: Where each protein sits in the body:
Keratin forms the outer protective layer of the body. Myosin is inside muscle cells. Insulin travels in blood as a hormone. Legumelin is found in plant seeds. Only keratin is part of nails, so it is the answer.
Final Answer:
Option (B).
\[ \boxed{\text{Keratin (B)}} \]