Assertion: In adiabatic expansion, temperature of gas always decreases.
Reason: In adiabatic process exchange of heat is zero.

Options

A. (a) If both Assertion and Reason are true and the Reason is correct explanation of the Assertion.
B. (b) If both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
C. (c) If Assertion is true, but the Reason is false.
D. (d) If Assertion is false but the Reason is true.

Explanation

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

### Step 1: Understand the Assertion
The assertion states that "In adiabatic expansion, the temperature of the gas always decreases."

- In an adiabatic process, there is no heat exchange with the surroundings (Q = 0).
- When a gas expands adiabatically, it does work on the surroundings. According to the first law of thermodynamics, the change in internal energy (ΔU) of the gas is equal to the heat added to the system (Q) minus the work done by the system (W). Since Q = 0 in an adiabatic process, we have:

\[ \Delta U = -W \]

- For an ideal gas, the change in internal energy is related to the change in temperature (ΔT) by the equation:

\[ \Delta U = nC_v\Delta T \]

- If the gas does work on the surroundings (W > 0), then ΔU must be negative, which implies that ΔT is also negative. Therefore, the temperature of the gas decreases during adiabatic expansion.

### Step 2: Understand the Reason
The reason states that "In an adiabatic process, the exchange of heat is zero."

- This is a fundamental characteristic of adiabatic processes. By definition, an adiabatic process is one in which no heat is transferred into or out of the system (Q = 0).

### Step 3: Conclusion
Both the assertion and the reason are true. The reason correctly explains why the assertion is true. Therefore, we can conclude that:

- **Assertion is true:** In adiabatic expansion, the temperature of the gas always decreases.
- **Reason is true:** In an adiabatic process, the exchange of heat is zero.

### Final Answer
Both the assertion and the reason are true, and the reason is the correct explanation of the assertion.