The statement that is not correct is:
A.Van der Waals constant ‘a’ measures extent of intermolecular attractive forces for real gases
B.Boyle point depends on the nature of real gas
C.Compressibility factor measures the deviation of real gas from ideal behaviour
D.Critical temperature is the lowest temperature at which liquefaction of a gas takes place

Hint: Since this is a fact - based question, let us first discuss the concepts mentioned here, and then derive conclusions for finding the correct answer. This would help us to understand the basics and make a sound decision while eliminating the wrong options.

Complete Step-by-Step Answer:
The options that are given to us, can be shortly described as follows:
1.Van der Waals constant: The Van der Waals equation of state is a modified version of the ideal gas law, which takes into consideration the interactive forces of the molecules and the size of the molecules. This equation can be represented as:
 \[[P + a{(\dfrac{n}{v})^2}](\dfrac{V}{n} - b) = RT\] ; where a and b are constants.
Here the constant value ‘a’ is positive and provides a correction for the intermolecular forces.
2.Boyle Point: Boyle point is the temperature at which a real gas begins to exhibit the properties of an ideal gas. It is a characteristic property and is dependent on the nature of the real gas.
3.Compressibility factor: Compressibility factor or compression factor can be explained as the ratio of molar volume of an ideal gas to the molar volume of a real gas. In simpler terms, it basically measures how far-off are the values from ideal gases to real gases.
4.Critical temperature: Critical temperature can be basically explained as the temperature above which a gas cannot be liquefied. This means that it is the highest at which the given gas can be liquefied.
From the above data, we can say that the statement in option D is not correct.
Hence, Option D is the correct option.

Note: In the Van der Waals state equation, the constants ‘a’ and ‘b’ have positive values and are characteristic of the individual gas. This equation approaches the ideal gas law as the values of these constants approaches zero.