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 (`A`) and (`R`) are correct and (`R`) is the correct explanation of (`A`).
B. If both (`A`) and (`R`) are correct, but (`R`) is not the correct explanation of (`A`).
C. If (`A`) is correct, but (`R`) is incorrect.
D. If (`A`) is incorrect, but (`R`) is correct.

Explanation

To solve the given assertion-reason question, we will analyze both the assertion and the reason step by step.

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

- We start with the ideal gas law, which is given by the equation:
\[ PV = nRT \]
where \( P \) is pressure, \( V \) is volume, \( n \) is the number of moles, \( R \) is the ideal gas constant, and \( T \) is temperature.

### Step 2: Relating Density to Pressure
- We can express the number of moles \( n \) in terms of mass \( m \) and molar mass \( M \):
\[ n = \frac{m}{M} \]
- Substituting this into the ideal gas equation gives:
\[ PV = \frac{m}{M}RT \]
- Rearranging for volume \( V \):
\[ V = \frac{mRT}{PM} \]

### Step 3: Expressing Density
- Density \( \rho \) is defined as:
\[ \rho = \frac{m}{V} \]
- Substituting the expression for \( V \) into the density equation:
\[ \rho = \frac{m}{\frac{mRT}{PM}} = \frac{PM}{RT} \]

### Step 4: Analyzing the Relationship
- From the derived equation \( \rho = \frac{PM}{RT} \), we can see that at constant temperature \( T \) and constant molar mass \( M \), density \( \rho \) is directly proportional to pressure \( P \):
\[ \rho \propto P \]
- Therefore, if pressure \( P \) is doubled, density \( \rho \) will also double.

### Conclusion for Assertion
- The assertion is **true**.

### Step 5: Understanding the Reason
The reason states: "At constant temperature, molecular mass of a gas is directly proportional to the density and inversely proportional to the pressure."

- We can express molecular mass \( M \) in terms of density \( \rho \) and pressure \( P \):
\[ M = \frac{\rho RT}{P} \]
- From this equation, we can see that at constant temperature \( T \), molecular mass \( M \) is indeed directly proportional to density \( \rho \) and inversely proportional to pressure \( P \).

### Conclusion for Reason
- The reason is **true**, but it does not fully explain the assertion because it does not mention that the molecular mass \( M \) is constant in this context.

### Final Conclusion
- Both the assertion and reason are correct, but the reason does not adequately explain the assertion without stating that the molecular mass is constant.

### Answer
- The correct answer is: **B** - Both are correct, but the reason is not a correct explanation of the assertion.