Assertion:- If barometer is acceleratated upwards, the level of mercury in the tube of barometer will decrease.
Reason:- The effective value of g will increase, so upthrust increase.

Options

A. If both Assertion `&` Reason are True `&` the Reason is a correct explanation of the Assertion.
B. If both Assertion `&` Reason are True but Reason is not a correct explanation of the Assertion.
C. If Assertion is True but the Reason is False.
D. If both Assertion `&` Reason are False.

Answer

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

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that if a barometer is accelerated upwards, the level of mercury in the tube will decrease.
- Normally, the height of mercury in a barometer is determined by the atmospheric pressure acting on the surface of the mercury in the reservoir.

2. **Effect of Upward Acceleration**:
- When the barometer is accelerated upwards, the effective gravitational acceleration acting on the mercury changes.
- The effective gravitational acceleration \( g_{\text{effective}} \) can be expressed as:
\[ g_{\text{effective}} = g + a \]
where \( g \) is the acceleration due to gravity and \( a \) is the upward acceleration of the barometer.

3. **Pressure Relationship**:
- The pressure at the base of the mercury column in the barometer is given by:
\[ P_{\text{ATM}} = \rho g H_0 \]
where \( H_0 \) is the original height of the mercury column, and \( \rho \) is the density of mercury.

4. **New Height of Mercury**:
- When the barometer is accelerated upwards, the new height \( H' \) of the mercury column can be expressed as:
\[ P_{\text{ATM}} = \rho g_{\text{effective}} H' \]
- Setting the two expressions for atmospheric pressure equal gives:
\[ \rho g H_0 = \rho (g + a) H' \]
- Simplifying this, we find:
\[ H' = \frac{g}{g + a} H_0 \]
- Since \( g + a > g \), it follows that \( H' < H_0 \). Therefore, the height of the mercury column decreases when the barometer is accelerated upwards.

5. **Understanding the Reason**:
- The reason states that the effective value of \( g \) will increase, so the upthrust increases.
- While it is true that the effective gravitational force increases, the reason does not correctly explain why the mercury level decreases. The decrease in height is due to the increase in effective gravitational acceleration leading to a decrease in the height of the mercury column, not due to an increase in upthrust.

6. **Conclusion**:
- The assertion is true: the level of mercury decreases when the barometer is accelerated upwards.
- The reason is not a correct explanation for the assertion.

### Final Answer:
- **Assertion**: True
- **Reason**: False