Phototropic movement is the movement of plants in response to light. The correct answer is option 1, auxin.

Auxin is a plant hormone responsible for several plant growth processes including phototropism. It accumulates on the shaded side of plant stems or shoots when exposed to light.

When light hits the stem or shoot, auxin is redistributed, resulting in increased growth on the shaded side and bending of the plant towards the light source. This movement enables the plant to optimize its exposure to light for photosynthesis.

Gibberellin, cytokinin, and ethylene are also plant hormones but they do not directly regulate phototropic movement.