Auxin affects the growth of plant stems by building up on the side of the stem that is away from the light source and stimulating growth. This differential distribution of auxin causes the cells on the shaded side of the stem to elongate more than those on the light-exposed side, resulting in the stem bending towards the light. Therefore, the correct response is:
It builds up on the side of the stem that is away from the light source and stimulates growth.