The component of a muscle fibre known as sarcolemma functions as :

Options

A. a contractile myofilament
B. a chemical at the synapse
C. a cross bridge unit on the myosin
D. a plasma membrane covering each cell

Explanation

To answer the question about the function of the sarcolemma in a muscle fiber, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Sarcolemma**: The sarcolemma is a specific component of muscle fibers. It is important to understand what it is and where it is located.

2. **Understand the Structure of Sarcolemma**: The sarcolemma is composed of a plasma membrane that surrounds each muscle fiber. It also includes a basement membrane and a layer of reticular connective tissue.

3. **Function of Sarcolemma**: The primary function of the sarcolemma is to act as a barrier and to regulate the movement of substances in and out of the muscle cell. This makes it similar to the plasma membrane found in other types of cells.

4. **Evaluate the Options**: The question provides four options regarding the function of the sarcolemma. We need to analyze each option:
- **Option 1**: Contractile myofilament - This is incorrect as myofilaments are structures within the muscle that facilitate contraction, not the function of the sarcolemma.
- **Option 2**: Chemical at the synapse - This does not relate to the sarcolemma's function.
- **Option 3**: Cross bridge unit on the myosin - This refers to a mechanism of muscle contraction, not the sarcolemma.
- **Option 4**: Plasma membrane governing each cell - This accurately describes the function of the sarcolemma.

5. **Conclusion**: Based on the analysis, the correct answer is that the sarcolemma functions as a plasma membrane governing each muscle cell.

### Final Answer:
The component of a muscle fiber known as sarcolemma functions as a plasma membrane governing each cell.