Which of the following setups can make a small conductor coil that is placed horizontally float on top of an object ?

Options

A. The object is a magnet that provides a magnetic field pointing straight downward, whereas the coil carries an electric current flowing clockwise as viewed from top down
B. The object is a magnet that provides a magnetic field pointing straight downward, while the coil carries an electric current flowing counterclockwise as viewed from top down
C. The object carries positive electric charge while the coil carries negative charge
D. The object as well as coil carries negative electric charge

Answer

To determine which setup can make a small conductor coil float horizontally on top of an object, we need to analyze each of the given setups based on the principles of magnetism and electric charge interactions.

### Step-by-Step Solution:

1. **Understanding the Problem**: We need to find a setup where a small conductor coil can float horizontally above an object. This implies there must be a force acting on the coil that counteracts gravity.

2. **Analyzing the First Setup**:
- The object is a magnet providing a magnetic field pointing straight downward.
- The coil carries an electric current flowing clockwise as viewed from the top.
- **Conclusion**: A current-carrying coil in a magnetic field experiences a magnetic force. However, since the magnetic field is downward and the current is clockwise, the force on the coil will not be upward. Therefore, this setup will not make the coil float.

3. **Analyzing the Second Setup**:
- The object is again a magnet providing a magnetic field pointing straight downward.
- The coil carries an electric current flowing counterclockwise.
- **Conclusion**: Similar to the first setup, the magnetic force acting on the coil will not be upward due to the direction of the current and magnetic field. Thus, this setup will also not make the coil float.

4. **Analyzing the Third Setup**:
- The object carries a positive electric charge.
- The coil carries a negative electric charge.
- **Conclusion**: Opposite charges attract each other. Therefore, the negative charge in the coil will be attracted to the positive charge of the object. This attraction will not allow the coil to float; instead, it will move towards the object.

5. **Analyzing the Fourth Setup**:
- The object and the coil both carry negative electric charges.
- **Conclusion**: Like charges repel each other. Therefore, the negative charge of the coil will repel the negative charge of the object, which can create a situation where the coil can float above the object due to the repulsive force.

6. **Final Answer**: Based on the analysis, the correct setup that can make the small conductor coil float horizontally on top of an object is the fourth setup, where both the object and the coil carry negative electric charges.

### Summary:
The correct answer is **Option 4**: The object as well as the coil carries the negative electric charge.