When an object is in equilibrium state, then -

Options

A. It must be at rest
B. No force is acting on it
C. Its net acceleration must be zero
D. All of these

Answer

To solve the question "When an object is in equilibrium state, then -", we need to analyze the conditions of equilibrium based on Newton's laws of motion.

### Step-by-Step Solution:

1. **Understanding Equilibrium**:
An object is said to be in equilibrium when the net force acting on it is zero. This means that all the forces acting on the object balance each other out.

**Hint**: Recall that equilibrium is defined by the condition of forces balancing each other.

2. **Types of Equilibrium**:
- **Static Equilibrium**: The object is at rest, and the net force is zero.
- **Dynamic Equilibrium**: The object is in motion with a constant velocity, and the net force is still zero.

**Hint**: Consider both static and dynamic scenarios when thinking about equilibrium.

3. **Analyzing the Options**:
- **Option 1: It must be at rest**: This is incorrect because an object can be in motion and still be in equilibrium (dynamic equilibrium).
- **Option 2: No force is acting on it**: This is also incorrect. Forces can act on an object in equilibrium, but they must balance out to result in a net force of zero.
- **Option 3: Its net force must be zero**: This is correct. For an object to be in equilibrium, the net force acting on it must be zero.
- **Option 4: All of these**: This is incorrect since not all previous statements are true.

**Hint**: Evaluate each option carefully to determine which statements align with the definition of equilibrium.

4. **Conclusion**:
The correct answer is that for an object to be in equilibrium, its net force must be zero. This means that while forces may be acting on the object, they balance out, resulting in no change in motion.

**Final Answer**: The correct option is that "its net force must be zero."