The bending of the bean seedling is most likely due to phototropism (Option B). Phototropism is the growth of a plant in response to light, where parts of the plant grow towards the light source. If the seedling is bending towards light, that is the phenomenon at play.
Gravitropism (Option A) involves growth in response to gravity, which would cause roots to grow downward and stems to grow upward. Thigmotropism (Option C) is growth in response to touch, typically seen in climbing plants. Rapid response (Option D) is not a specific term generally used in plant growth.
Based on the information provided, the correct answer is B. phototropism.