The correct option is A Both assertion and reason are true and reason is the correct explanation of assertion.
The ease of liquefaction of a gas depends on their inter-molecular force of attraction which in turn is measured in terms of van der Waals constant ‘a’. Hence, the higher the value of ‘a’, greater the inter-molecular force of attraction, easier the liquefaction.
Hence, both assertion and reason are true and the reason is the correct explanation of assertion.