Plant movement in response to diffuse stimulus of light is
a. Phototropism
b. Photolysis
c. Phototaxis
d. Photonasty

Hint: Plant movement in response to diffuse stimulus of light is due to a chemical called auxin in the plants. Sometimes the plant moves towards the stimulus of the light and sometimes away from the stimulus of the light.

Complete answer:
> Option A, Phototropism is the growth of the plants or other organism in the response to the stimulus of the light. The cells which are present at the furthest side of the source of light elongate and the plant moves towards the stimulus of the light.

> Option B, Photolysis is defined as decomposition or splitting of the chemical compounds in an organism. In the process of photosynthesis the water photolysis in the presence of sunlight into oxygen and hydrogen compounds.

> Option C, Phototaxis is the movement of an organism towards or away from the stimulus of the light. If an organism moves towards the stimulus of increasing intensity of light it is known as positive phototaxis and if it moves away from stimulus of increasing intensity of light it is called negative phototaxis. In this plants orient themselves to high intensity of light to receive efficient light for photosynthesis.

> Option D, Photonasty is defined as independent movement of an organism with the stimulus of light without the effect of position of stimulus.

Hence, The correct answer is option (C).

Note: All the above options are associated with the light source but the effect of light on each of the above options is different. All the options refer to movement of organisms or compounds towards the light but each of the options give a specific nature of its movement under certain conditions towards the stimulus of light.