Phototropic movement shown by the shoot are due to:
Auxin plays an essential role in phototropism. Depending on the direction of light stimulus plants begin redistribution of the auxin molecules to the farthest side from the light, causing the cells on the dark side to grow larger than corresponding cells on the illuminated side of the plant. This produces a curving of the plant stem tip toward the light, a plant movement known as phototropism.
Thus, the correct answer is option A.