(i) Water will accumulate in the hollowed portions of B and C as a result of difference in the water concentration, due to which a concentration gradient forms and thus water from higher concentration (from the trough containing water) flows to a region of lower water concentration (hollowed portion of potato cups). This is a result of osmosis of water in a hypotonic solution.
(ii) Potato cup A is essential in this experiment as it is a significant reference to compare different scenarios seen in potato cups B, C and D.
(iii) Potato cup A didn't show any change in the concentration because it is an example of isotonic solution. Potato cup D is a boiled potato which tells us that this potato has same or more water concentration than the trough containing water in which potato cup D is placed. So there is no flow of water from the trough into the hollow portion of potato cup D.