Final answer:
The purpose of placing pot A in the dark is to serve as a control to observe the effects of darkness on the growth of seedlings, allowing for a comparison with seedlings in different light conditions.
Step-by-step explanation:
In the experiment with three pots labeled A, B, and C containing 3-day-old cowpea seedlings, the purpose of placing pot A in a dark box or cupboard is to observe the effect of darkness on seedling growth.
This arrangement is a classic scientific method of using a control setup wherein pot A would serve as the control to compare the growth of seedlings in darkness to those exposed to different light conditions in pots B and C. By doing so, researchers can isolate the variable of light and better understand its impact on plant growth processes such as germination and photosynthesis.