The answer is Option A - Phototropism.

The phenomenon of phototropism is the growth of organisms in response to light. Charles Darwin and his son Francis Darwin were among the early scientists to study phototropism. They found that light is perceived by the tip of the plant shoot, and this perception leads to growth further down the shoot.

Later, it was discovered that a hormone, named auxin, is responsible for this effect. Auxin accumulates on the side of the plant facing away from the light source, and causes cells on that side to elongate, thus bending the plant towards the light. This played a major role in the discovery of auxin, which is one of the main plant hormones.

While auxins do also influence other processes like root initiation, gravitropism, and apical dominance, their role in phototropism was significant to their discovery.

So, based on your question, phototropism (Option A) is the correct answer.