Select incorrect statement :

Options

A. we can condense vapour simply by applying pressure
B. to liquefy a gas one must lower the temperature below `T_(C)` and also apply pressure
C. at `T_(C)` there is no distinction between liquid and vapour state hence density of the liquid is nearly equal to density of the vapour
D. However great the pressure applied, a gas cannot be liquified below it's critical temp.

Explanation

To solve the question of selecting the incorrect statement regarding the gaseous state, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement A: "We can condense vapor simply by applying pressure."**
- This statement is **true**. Condensation of vapor can occur by applying pressure, which forces the gas molecules closer together, allowing them to transition into the liquid state.

2. **Statement B: "To liquefy a gas, we must decrease temperature below Tc (critical temperature) and increase its pressure."**
- This statement is also **true**. Liquefaction of a gas requires that the temperature be lowered below the critical temperature (Tc) and that pressure be applied to facilitate the transition from gas to liquid.

3. **Statement C: "At critical temperature, there is no distinction between liquid and vapor state; hence the density of liquid is nearly equal to density of vapor."**
- This statement is **true**. At the critical temperature, the properties of the liquid and vapor phases converge, resulting in nearly equal densities for both states.

4. **Statement D: "However, great pressure applied, a gas cannot be liquefied below its critical temperature."**
- This statement is **false**. A gas can indeed be liquefied below its critical temperature if sufficient pressure is applied. Therefore, this statement is incorrect.

### Conclusion:
The incorrect statement is **D**.