Final answer:
In response to the student's question, light stimulus causes an organism to exhibit phototaxis, which is the directed movement towards or away from a light source.
Step-by-step explanation:
The student's question, "Light stimulus causes ____A. phototaxis B. chemotaxis," is aimed at identifying the type of movement response that is triggered by a light stimulus. In biological terms, when an organism moves toward or away from a light source, this directed movement is known as phototaxis. In contrast, chemotaxis refers to the movement toward or away from a chemical signal. An organism demonstrates a positive taxis when it moves toward a stimulus, and a negative taxis when it moves away from it. For example, bacteria may exhibit phototaxis by moving towards light, which is a positive phototactic response.