Commercial plant growers artificially control the lengths of days and nights. this produces a response called
a. photoperiodism.
b. gravitropism.
c. thigmotropism.
d. phototropism.

The answer is A. photoperiodism.

Photoperiodism is an organism reaction to the lengths of days and nights. Some plants require longer days, and some require shorter days. By changing the lengths of days and nights, commercial plant growers are able to increase plants productivity.

Tropism is plant growth in a response to a stimulus.
As it name suggests, phototropism is plant growth in a response to light. 
Gravitropism is plant growth in a response to gravity.
Thigmotropism is plant growth in a response to contact

When you are making a guess you are forming a hypothesis