Reason for the same level of water in cup A:

1. There is no solution/solute inside cup A.
2. So, osmotic pressure will be absent in this case.
3. Hence, the water does not move from outside to inside of the cup.

Reason for increased water level in cup B:

1. Osmosis is the reason for the increase of water level in cup B.
2. Osmosis is the movement of the solvent molecules through a semipermeable membrane from a region of low solute concentration to a region of higher solute concentration.
3. Here the cells of the potato act as the semi-permeable membrane through which water enters the potato cup.
4. The medium inside the potato cup has a higher concentration of sugar (solute) as compared to outside the potato cups.
5. Hence, the water moves towards the internal part of the potato cup which is the movement of solvent from a region of low solute concentration to a region of higher solute concentration.

Reason for increased water level in cup C:

1. Osmosis is the reason for the increase of water level in cup B.
2. Osmosis is the movement of the solvent molecules through a semipermeable membrane from a region of low solute concentration to a region of higher solute concentration.
3. Here the cells of the potato act as the semi-permeable membrane through which water enters the potato cup.
4. The medium inside the potato cup has a higher concentration of salt (solute) as compared to outside the potato cups.
5. Hence, the water moves towards the internal part of the potato cup which is the movement of solvent from a region of low solute concentration to a region of higher solute concentration.

Reason for the same level of water in cup D:

1. Cup D is made up of boiled potato.
2. The cells of the boiled potato are dead including their cell membranes.
3. Therefore, there is no membrane across which the osmosis can take place.
4. Hence, there is no change in the level of water inside cup D.

Final Answer: Hence, water gathers in the hollowed portions of the potato cup B and C due to the occurrence of osmosis.