Which of the following structures contract and relax rhythically to produce movement ?

Options

A. Flagella
B. Cilia
C. Muscles
D. Pseudopodia

Answer

To determine which structures contract and relax rhythmically to produce movement, we need to analyze the given options: flagella, cilia, muscles, and pseudopodia.

### Step-by-Step Solution:

1. **Understanding the Options**:
- **Flagella**: These are long, whip-like structures that help in locomotion, primarily in prokaryotic organisms (like bacteria). They do not contract and relax in a rhythmic manner like muscles do.
- **Cilia**: These are short, hair-like structures found on the surface of some eukaryotic cells. They also aid in movement but do not contract and relax rhythmically in the same way that muscles do.
- **Muscles**: Muscles are specialized tissues in the body that contract and relax rhythmically. This contraction and relaxation is what enables movement in multicellular organisms, including humans.
- **Pseudopodia**: These are temporary projections of eukaryotic cells, like amoeba, used for movement and feeding. They do not contract and relax rhythmically.

2. **Identifying the Correct Answer**:
- Among the options, only **muscles** are responsible for contraction and relaxation that leads to movement. Therefore, the correct answer is **muscles**.

### Final Answer:
The structure that contracts and relaxes rhythmically to produce movement is **muscles**.

---