In the following question, select the related number from the given alternatives.
Sapling : Tree :: Girl : ?

Given: Sapling : Tree :: Girl : ?

A sapling is a young form of a tr​ee.

So, the relationship is:
Young stage → Grown stage

Now apply the same logic to:
Girl : ?

A girl is a young form of a woman.

(Just as a sapling grows into a tree, a girl grows into a woman.)

Thus, correct option is (a).