The downward growth of a root is an example of
a. phototropism
b. photoperodism
c. negative gravitropism
d. positive gravitropism
D?
Unless you have typos, it is none of above.
Geotropism. Plants can sense the Earth's gravitational field. Geotropism is the term applied to the consequent orientation response of growing plant parts. Roots are positively geotropic, that is, they will bend and grow downwards, towards the center of the Earth.