The answer is A. Sky : Cloud.
The other options are all pairs of words that are synonyms. A sky is a cloudless expanse of air, while a cloud is a visible mass of condensed water droplets or ice particles in the atmosphere above the Earth. A purse is a small bag for carrying money, while a wallet is a flat, folding case for carrying money, credit cards, and other personal items. A cupboard is a tall, freestanding cabinet with doors, while an almirah is a large, freestanding cabinet with doors, typically used for storing clothes. A chair is a piece of furniture with a seat and back, while a stool is a small, low seat without a back.
In the pair sky : cloud, the two words are not synonyms. A sky is not a type of cloud. A sky is the background against which clouds appear.