A sarcomere is :

Options

A. the area between two intercalated discs
B. the nonfunctional unit of skeletal muscle
C. the wavy lines on the cell, as seen in a microscope
D. the contractile unit between two Z disc

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the definition of a sarcomere. We need to identify what a sarcomere is in the context of muscle anatomy.

2. **Analyzing the Options**:
- **Option A**: "The area between two intercalated discs."
- This is incorrect. Intercalated discs are found in cardiac muscle, not related to the definition of a sarcomere.
- **Option B**: "The non-functional unit of a skeletal muscle."
- This is also incorrect. A sarcomere is indeed a functional unit of skeletal muscle.
- **Option C**: "The wavy lines on the cells as seen in the microscope."
- This statement is incorrect as well. Wavy lines do not define a sarcomere.
- **Option D**: "The contractile unit between two Z discs."
- This is the correct definition of a sarcomere. It is indeed the contractile unit of muscle fibers.

3. **Defining Sarcomere**: A sarcomere is the basic contractile unit of muscle fibers. It is responsible for muscle contraction and is composed of two main protein filaments: actin and myosin.

4. **Function of Sarcomere**: The primary function of a sarcomere is to facilitate movement through contraction. The interaction between actin and myosin filaments leads to the shortening of the sarcomere, resulting in muscle contraction.

5. **Conclusion**: Based on the analysis of the options and the definition provided, the correct answer is that a sarcomere is the contractile unit between two Z discs.

### Final Answer:
A sarcomere is the contractile unit between two Z discs.

---