Select the option that is related to the third word on the same basis as the second word is related to the first word.
Glass : transparent :: Wood : ?

Given: Glass : transparent :: Wood : ?

The word "transparent" describes a property or characteristic of glass.

Now apply the same logic to:

Wood : ?

This is the opposite of transparent and is a natural property of wood.

Wood is naturally opaque, wood is known for being opaque.

Thus, correct option is (a).