Answer:
c. the plant root growing down into the soil
Explanation:
The plant root going down deep into the soil is an example of positive gravitropism. Gravity pulls everything towards the center of earth i.e. downwards and roots follow the direction of gravity automatically by growing downward towards soil that is why it is known as positive gravitropism. In short we can say that root favors the direction of gravity.
Shoot on the other hand show negative geotropism/gravitropism i.e. it grows towards the opposite direction of gravity. Shoot grow towards upside direction i.e. away from the surface towards sky.
