Number analogy section deals with two types of questions:
I. Choosing a similarly related pair as the given number pair on the basis of the relation between the numbers in each pair.
II. Choosing a number similar to a group of numbers on the basis of certain common properties that they possess.
324 : 162
Clearly, the relationship is 2x : x.
This relationship exists in (b).
So, the correct answer is (b).