The correct answer is A. Sky : Cloud.
In the other options, the first word is a container and the second word is an object that is typically stored in the container. For example, a purse is a container for money and other personal items, and a wallet is a container for money and credit cards. A cupboard is a container for food and other household items, and an almirah is a container for clothes and other personal items. A chair is a piece of furniture for sitting on, and a stool is a smaller piece of furniture for sitting on.
In the case of sky and cloud, the sky is not a container for clouds. Clouds are objects that float in the sky.