A: If a toroid uses bismuth for its core, then the field in the core will be slightly less than when the core is empty.
R: Bismuth is paramagnetic.

Options

A. If both Assertion & Reason are the true and the reason is the correct explanation of the assertion, then mark (1)
B. If both Assertion & Reason are true but the reson is not the correct explanation of the assertion, then mark (2)
C. If Assertion is true statement but Reason is false, then mark (3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that if a toroid uses bismuth for its core, then the magnetic field in the core will be slightly less than when the core is empty.
- This implies that the presence of bismuth affects the magnetic field strength in the core.

2. **Understanding the Reason (R)**:
- The reason states that bismuth is paramagnetic.
- Paramagnetic materials have a tendency to become magnetized in the direction of an external magnetic field, which would typically increase the magnetic field strength in the core.

3. **Analyzing the Properties of Bismuth**:
- Bismuth is actually a diamagnetic material, not paramagnetic. Diamagnetic materials are characterized by their ability to create an induced magnetic field in the opposite direction to an applied magnetic field.
- This means that when bismuth is placed in a magnetic field, it will slightly weaken the magnetic field inside the core.

4. **Conclusion**:
- Since bismuth is diamagnetic, the assertion (A) is correct: the magnetic field in the core will indeed be slightly less than when the core is empty.
- However, the reason (R) is incorrect because it incorrectly classifies bismuth as paramagnetic instead of diamagnetic.

5. **Final Answer**:
- The assertion (A) is true, but the reason (R) is false. Therefore, the correct answer is that A is true and R is false.