A solar water heater cannot be used to get hot water on
$(a)$ a sunny day
$(b)$ a cloudy day
$(c)$ a hot day
$(d)$ a windy day

Correct answer is: $(b)$ a cloudy day

Explanation:

A solar water heater is a device that converts solar energy into thermal energy. Without solar energy, it can't work. If a solar heater is used without sunlight, it will not work at all.

On a cloudy day:

A solar water heater cannot be used to get hot water on a cloudy day because it receives energy from the sun, and on a cloudy day, the sun does not appear.

On a sunny day:

Whereas on a sunny day the sun appears and a solar water heater converts sunlight energy into heat energy efficiently on a sunny day.

On a hot day:

The day can't get hot if the sun isn't appearing. a solar heater can be used on a hot day as there is a great chance of the presence of sunlight on a hot day.

On a windy day:

When the sun's sunlight falls on the earth, the land and the air there start heating up. Warm air is lighter than cold air, so it tends to rise upwards and the air pressure is decreased. Due to this air starts coming from the colder places of high pressure. This is said to be the wind blowing. So sunlight causes the wind to blow.

Therefore, a solar heater can be used on a windy day too.