Correct option is B. Brushing teeth
Amount of water needed for given activities as
A) Bathing needs at least a bucket of water or a little more water.
B) Brushing teeth needs a mug of water.
C) The amount of water needed to wash clothes depends upon the number of clothes but usually, it is considered as more than a bucket of water is needed.
D) Mopping a room needs more water than brushing teeth.
$$\textbf{Conclusion:}$$ Brushing teeth activity needs less amount water and hence it is the correct answer