Astem growing up and out of the soil, away from gravity is an example of a
a. positive tropism
b. negative tropism
c. negative phototropism
d. positive gravitropism

A tropism is a movement of an organism toward or away from a stimulus. A positive tropism is when the organism moves toward the stimuli. A negative tropism is when the organism moves away from the stimuli. So, your answer will be negative tropism, since the stem is growing up and out of the soil, AWAY from gravity.

The answer is constant.