Assertion: If a body is momentarily at rest, it means that force or acceleration are necessarily zero at that instant.
Reason: Force on a body at a given time is determined by the direction of motin only.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true and reason is the not correct explanation of assertion.
C. If assertion is true but reason is false
D. If both assertion and reason are false.

Explanation

To solve the given question, we need to analyze both the assertion and the reason provided.

### Step 1: Analyze the Assertion
The assertion states: "If a body is momentarily at rest, it means that force or acceleration are necessarily zero at that instant."

- A body being momentarily at rest indicates that its velocity is zero at that instant. However, this does not imply that the forces acting on the body are zero.
- For example, consider an object hanging from a string. At the moment it is at rest, the gravitational force acting downwards is balanced by the tension in the string acting upwards. Thus, the net force is zero, but the individual forces (gravitational force and tension) are not zero.
- Acceleration is indeed zero when the body is at rest, but the forces can still be present. Therefore, the assertion is **false**.

### Step 2: Analyze the Reason
The reason states: "Force on a body at a given time is determined by the direction of motion only."

- This statement is also incorrect. The force acting on a body is not solely determined by its direction of motion.
- For example, in the case of a body moving in the opposite direction (retarding motion), the force acting on the body can be in the opposite direction to its motion. This indicates that the direction of motion does not solely dictate the force acting on the body.
- Therefore, the reason is also **false**.

### Conclusion
Both the assertion and the reason are false.

### Final Answer
Both assertion and reason are false.

---