An air-pump is kept in an aquarium in order to

Correct option is B

The correct answer is allow more oxygen to dissolve in water.
An air pump simply bubbles air through the tank. Air pumps serve two purposes. First, they ensure that the tank maintains an adequate concentration of oxygen. An air pump is not required for this purpose, as long as your tank maintains adequate water movement together with surface agitation. Fish breathe in oxygen dissolved in water.