Explanation:
The Van der Waals equation is an improvement of the ideal gas law that takes into account the non-ideal behavior of real gases. It introduces two correction factors, a and b, known as Van der Waals constants.

The Van der Waals constant 'a' accounts for the intermolecular forces between gas molecules, which are attractive in nature. A higher value of 'a' indicates stronger intermolecular forces and therefore a greater tendency for the gas to be liquefied.

Gas can be easily liquefied:
When the value of 'a' is higher, it means that the gas molecules experience stronger attractive forces. These forces tend to bring the gas molecules closer together, increasing the likelihood of interactions and collisions. As a result, the gas molecules are more likely to condense and form a liquid phase.

In contrast, a lower value of 'a' indicates weaker intermolecular forces and a lower tendency for the gas to be liquefied. The gas molecules are more likely to remain in the gaseous state and exhibit behavior closer to that of an ideal gas.

Effect of other options:
- Molecules of the gas have smaller diameter: The Van der Waals constant 'b' takes into account the volume occupied by the gas molecules themselves. It represents the excluded volume or the volume that cannot be occupied by other molecules. The size of the gas molecules is accounted for by 'b', not 'a'. Therefore, the diameter of the gas molecules does not affect the value of 'a' or the ease of liquefaction.
- Gas has higher molecular weight: The molecular weight of a gas does not directly affect the Van der Waals constant 'a'. The value of 'a' depends on the strength of the intermolecular forces, which can vary for gases with different molecular weights.
- Gas has lower molecular weight: Similar to the previous option, the molecular weight of a gas does not directly affect the Van der Waals constant 'a'. The ease of liquefaction depends on the strength of the intermolecular forces, not the molecular weight.

Therefore, the correct answer is option 'B' - a higher value of Van der Waals constant 'a' indicates that the gas can be easily liquefied due to stronger intermolecular forces.