Assertion: At constant temperature, if pressure on the gas is doubled, density is also doubled.
Reason: At constant temperature, molecular mass of a gas is directly proportional to the density and inversely proportional to the pressure

Options

A. If both assertion and reason are correct and reason is correct explanation for assertion
B. If both assertion and reason are correct and reason is not correct explanation for assertion
C. If assertion is correct but reason is incorrect
D. If assertion and reason are both incorreect.

Explanation

To analyze the assertion and reason provided in the question, we will break down the concepts step by step.

### Step 1: Understanding the Assertion
The assertion states that at constant temperature, if the pressure on the gas is doubled, the density is also doubled.

**Explanation**:
- We know from the ideal gas law that density (D) of an ideal gas can be expressed as:
\[ D = \frac{P \cdot m}{R \cdot T} \]
where:
- \(D\) = density
- \(P\) = pressure
- \(m\) = molar mass
- \(R\) = gas constant
- \(T\) = temperature

- If the pressure \(P\) is doubled (i.e., \(P' = 2P\)) while keeping the temperature \(T\) constant, the new density \(D'\) becomes:
\[ D' = \frac{P' \cdot m}{R \cdot T} = \frac{2P \cdot m}{R \cdot T} = 2D \]
- This shows that the density indeed doubles when the pressure is doubled.

### Conclusion for Step 1:
The assertion is **true**.

### Step 2: Understanding the Reason
The reason states that at constant temperature, the molecular mass of a gas is directly proportional to the density and inversely proportional to the pressure.

**Explanation**:
- From the equation \(D = \frac{P \cdot m}{R \cdot T}\), we can rearrange it to express the relationship between density, pressure, and molar mass:
\[ m = \frac{D \cdot R \cdot T}{P} \]
- This indicates that:
- Molar mass \(m\) is directly proportional to density \(D\) (as \(P\) and \(R \cdot T\) are constants).
- Molar mass \(m\) is inversely proportional to pressure \(P\).

### Conclusion for Step 2:
The reason is **true** in terms of the relationship between molar mass, density, and pressure. However, the assertion that "molecular mass of a gas is directly proportional to density" can be misleading because it does not specify that this is true only when pressure is held constant.

### Final Conclusion:
- The assertion is **true**.
- The reason is **true**, but it can be misleading without proper context.

### Answer:
The assertion is correct, and the reason is also correct but needs clarification. Therefore, the answer is that the assertion is true, and the reason is true, but the reason does not fully justify the assertion.