Contractile protein is

Options

A. actin
B. myosin
C. tropnin
D. tropomyosin

Answer

### Step-by-Step Solution:

1. **Understanding Contractile Proteins**:
- Contractile proteins are proteins that are responsible for muscle contraction and movement in cells. They play a crucial role in the structure and function of muscle tissue.

2. **Identifying the Options**:
- The options given are:
1. Actin
2. Myosin
3. Troponin
4. Tropomycin

3. **Analyzing Each Option**:
- **Actin**: This is a key contractile protein that forms thin filaments in muscle fibers. It exists in two forms: G-actin (globular actin) and F-actin (filamentous actin).
- **Myosin**: This is another important contractile protein that forms thick filaments and works alongside actin in muscle contraction.
- **Troponin**: This is a regulatory protein that binds to actin and tropomyosin, playing a role in muscle contraction but is not a contractile protein itself.
- **Tropomyosin**: This protein also regulates muscle contraction by blocking the binding sites on actin but is not contractile.

4. **Conclusion**:
- The primary contractile protein responsible for muscle contraction is **Actin**. Therefore, the correct answer is **Option 1: Actin**.