Extremely small pollen grains in water are viewed through a microscope. The grains are seen to move continually and randomly. What is the reason for this random movement?
(A) The grains are moved by randomly moving water molecules.
(B) The grains are moved by random convection currents in the water.
(C) The grains are moved by random rays of light reflecting off them.
(D) The grains are moved by the random motion of their own atoms.

Hint: We are given the situation in which the grains are viewed under the microscope and are told about the observations and are asked to find the reason behind the sort of observation. Thus, we will take into account all the possible types of fluid motions as the grains are placed in a fluid (water). Then, we will try to make a reasonable justification for the observation.

Complete step by step solution:
Here, we are given a situation where some microscopic particles are suspended in a fluid which is water in this case. The situation goes hand in hand with the basic requirement for Brownian motion to take place.
Now,
If we take Brownian motion as the explanation to the observation as per the question, it is fairly justified that the pollen grains will move. Moreover it will move erratically. Because as per definition, Brownian motion is the erratic (continuous but random) movement of microscopic particles suspended in a fluid. The motion is caused due to some disturbances in the medium of the suspended particles. In other words it is caused due to some disturbances in the fluid which might be caused due to some bombardments.

Hence, the correct explanation is given by (A).

Note: We discussed directly the Brownian motion as out of all the types of fluid motion, the given situation tallied with the fundamentals of Brownian motion. If the situation was different, then the explanation might also differ as it might take some other type of fluid motion into account.