Question

A wooden board, pipe${{P}_{1}}$, table and stop watch are arranged as shown in the figure. We are to fill the unknown boxes.

Answer 1

You could observe the given experimental setup more closely. You could thus try to identify the experiment that this setup is intended to or at least some hint of what it is related to. Thus, you could identify the things that correspond to the blank boxes given in the question.

Complete answer:
In the question, we are given a set up involving few equipment. With the look of it the setup seems to be that related to optics. In the figure we are already given some of the things already marked and we are supposed to find the ones that are marked blank.
We have a pipe ${{P}_{1}}$, a stopwatch, a table and a wooden board already marked.
The blank box A corresponds to a ray. If you observe the figure more closely we find that it is the incident ray.
The blank box B corresponds to some plain surface.
The blank box C corresponds to pipe 2. You could see that through it we have the reflected ray passing.
Now the fourth blank box D corresponds to the Normal. This could be confirmed by the fact that the incident angle and reflected angle are marked between incident ray/reflected ray with this line.

Note:
We have answered this question based on certain assumptions without any hint of what exactly this experimental setup is intended. We are given two angles marked as i and r and we have assumed them to be incident and reflected angle respectively. We cannot actually identify what the marked surface is just by looking at the figure.