Step 1: Understanding the Concept:
Muscles contract because of two proteins, actin and myosin. Myosin is the main structural and motor protein of muscle.
Step 2: Options:
Keratin is found in hair, nails and skin. Legumelin is a protein from legumes (pulses). Insulin is a hormone made by the pancreas. So myosin is the muscle protein.
Final Answer:
The muscle protein is myosin, option (B).
\[ \boxed{\text{Myosin}} \]