Ans.
Phototropism can be defined as a type of plant growth, in which light acts as a stimulus. Auxin is a plant hormone that promotes phototropism in plant parts.
Auxin is synthesized in the shoot tips in the presence of sunlight and is transported to the shaded side of shoot tip. Auxin increases cell division and elongation in shaded side and caused bending of plant toward the sunlight.
Thus, the correct answer to be filled in the blank is option). 'auxin.'