Plants bend toward the light
a.
response
b.
stimulus

Answer is option A.

The bending of plants is the response to the stimulus of light.

Explanation:

Phototropism is the growth response shown by plants in which they bend towards the light stimulus as light is the energy source for plant growth. The plant hormone, auxin also moves to the shaded side of the plant and causes more cell division and cell elongation on the darker side than the corresponding cells on the lighter side of the plant. This results in the curving of the plant stem toward the light.