Assertion `:-` At constant temperature `PV` vs `V` plot for real gas is not a straight line.
Reason `:-` At high pressure, all gases have `Zgt1` but at low pressure most gases have `Zlt1`

Options

A. Both A and R are true and R is the correct explanation of A
B. Both A and R are true but R is not the correct explanation of A
C. A is true but R is false
D. A is false but R is true

Explanation

To solve the assertion-reason question regarding the behavior of real gases, we will analyze both the assertion and the reason step by step.

### Step 1: Understand the Assertion
The assertion states that at constant temperature, the plot of \( PV \) versus \( V \) for real gases is not a straight line.

- For ideal gases, the relationship between pressure (P), volume (V), and temperature (T) is described by the ideal gas law \( PV = nRT \), which implies a linear relationship when plotted as \( PV \) versus \( V \).
- However, real gases deviate from this behavior due to intermolecular forces and the volume occupied by gas molecules.

**Conclusion for Step 1**: The assertion is true because real gases do not follow the ideal gas law perfectly, leading to a non-linear \( PV \) vs. \( V \) plot.

### Step 2: Understand the Reason
The reason provided states that at high pressure, all gases have \( Z > 1 \), but at low pressure, most gases have \( Z < 1 \).

- The compressibility factor \( Z \) is defined as \( Z = \frac{PV}{nRT} \).
- At high pressures, gases are compressed, and the volume of the gas molecules becomes significant compared to the total volume, leading to \( Z > 1 \).
- At low pressures, the intermolecular attractions become more significant, which can lead to \( Z < 1 \).

**Conclusion for Step 2**: The reason is also true, as it describes the behavior of real gases under different pressure conditions.

### Step 3: Analyze the Relationship Between Assertion and Reason
While both the assertion and reason are true, the reason does not provide a direct explanation for why the \( PV \) vs. \( V \) plot for real gases is not a straight line.

- The deviation of real gases from ideal behavior is primarily due to the presence of intermolecular forces and the finite volume of gas molecules, which is not addressed in the reason provided.

### Final Conclusion
Both the assertion and the reason are true, but the reason does not correctly explain the assertion. Therefore, the correct answer is that both assertion and reason are true, but the reason is not the correct explanation of the assertion.