A : `Fe_(3)O_(4)` is ferrimagnetic at room temperature but becomes paramagnetic at 850 K.
R : The magnetic moment in `Fe_(3)O_(4)` are aligned equally in parallel and antiparallel directions which on heating randomise.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion then mark (1)
B. If both Assertion & Reason are true but the reason 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 given question, we need to analyze the Assertion (A) and Reason (R) provided regarding the magnetic properties of `Fe_(3)O_(4)`.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that `Fe_(3)O_(4)` is ferrimagnetic at room temperature but becomes paramagnetic at 850 K.
- Ferrimagnetism occurs when there are unequal magnetic moments in opposite directions, leading to a net magnetic moment.

2. **Analyzing the Ferrimagnetic Nature**:
- In `Fe_(3)O_(4)`, there are two types of iron ions: Fe²⁺ and Fe³⁺.
- The magnetic moments of these ions are not equal; thus, they align in opposite directions (antiparallel) but do not completely cancel each other out, resulting in a net magnetic moment.

3. **Effect of Temperature**:
- As the temperature increases to 850 K, thermal energy disrupts the ordered arrangement of magnetic moments.
- This disruption leads to a randomization of the alignment, causing the material to lose its ferrimagnetic properties and become paramagnetic.

4. **Understanding the Reason (R)**:
- The reason states that the magnetic moments in `Fe_(3)O_(4)` are aligned equally in parallel and antiparallel directions, which randomize upon heating.
- This statement is incorrect because the magnetic moments are not aligned equally in parallel and antiparallel directions; they are primarily in antiparallel directions with unequal magnitudes.

5. **Conclusion**:
- The assertion is true, while the reason is false.
- Therefore, the correct conclusion is that the assertion is correct, but the reason provided is incorrect.

### Final Answer:
- The assertion (A) is true, and the reason (R) is false.