Answer: C. Auxin
Step-by-step explanation:
Phototropism can be define as the growth of the plant in response to the stimulus of light. The plant parts grow in direction of the sunlight this phenomena is called as positive phototropism. For example, stem. The plant parts when grow against the direction of sunlight this phenomena is called as negative phototropism. For example, root.
The auxin is a plant hormone which is responsible for phototropism. It moves to the darker side of the plant and helps the growth of the plant on that side as compared to the lighter side of the plant. This results in curving of the plant stem tip towards the light hence, results in positive phototropism.