Roots growing down into the ground are examples of

a. phototrophism
b. geotrophism
c. thigmotrophism
d. hydrotrophism

The correct answer for the blank is-

B. geotropism.

When plants respond towards gravitational force (that is gravity) and exhibit growth towards earth, it is known as gravitropism or geotropism (particularly positive geotropism).

In other words, growth of plant towards earth indicates geotropism.

For instance, whenever we see the plant roots growing towards ground, it depicts positive geotropism.

Thus, option B) is the right answer.