A force is needed to:

Options

A. change the state of motion or state of rest of the body
B. keep the body in motion
C. keep the body stationary
D. keep the velocity of body constant.

Explanation

To solve the question "A force is needed to:", we need to analyze the options provided and apply our understanding of Newton's laws of motion, particularly the concept of inertia.

### Step-by-Step Solution:

1. **Understanding the Concept of Force**:
- A force is an interaction that, when unopposed, will change the motion of an object. According to Newton's first law of motion, an object at rest stays at rest, and an object in motion stays in motion unless acted upon by a net external force.

2. **Analyzing the Options**:
- **Option A**: Change the state of motion or state of rest of the body.
- **Option B**: Keep the body in motion.
- **Option C**: Keep the body stationary.
- **Option D**: Keep the velocity of the body constant.

3. **Evaluating Each Option**:
- **Option A**: This option is correct because a force is indeed required to change the state of motion (from rest to motion) or to change the motion of a moving object (like speeding up, slowing down, or changing direction).
- **Option B**: This option is incorrect. If a body is already in motion, it will continue to be in motion unless a force acts on it to change that state.
- **Option C**: This option is also incorrect. A body at rest will remain at rest unless a force acts on it.
- **Option D**: This option is incorrect as well. A constant velocity means that there is no net force acting on the body (Newton's first law).

4. **Conclusion**:
- Based on the analysis, the only option that correctly describes the necessity of a force is **Option A**: "Change the state of motion or state of rest of the body."

### Final Answer:
The correct answer is **Option A**: Change the state of motion or state of rest of the body.

---