Auxins are synthesized at the shoot tip (apical meristem).
When light falls from one side, auxins migrate to the shaded side of the shoot. This causes cells on the shaded side to elongate more than those on the lit side, making the shoot bend towards the light — this is phototropism.
Examiners look for two things: (1) the site of synthesis and (2) the mechanism — unequal distribution of auxin causing differential cell elongation leading to bending. Both points must be mentioned to score full 2 marks. Avoid vague answers like "auxins cause growth"; be specific about unequal elongation.