Answer:B. speeds up the growth of the plant on the shaded side
Explanation: plants have hormones.these hormones controls the activities of the plants by serving as specific chemical signals between cells. They are usually moved from where they are produced to the region of the plant where they act. Eg auxin, cytokinin, gibberellin etc
Auxin has the ability to cause enlongation in shoot cells and root . They cause the cells of plant to stretch. they are produced in the apical meristem,buds and leaves. They also causes the suppression of side buds.
When a plant is placed in a dark shade and a side is exposed to light, auxin gathers on the shaded part of the plant. This causes the cells in that area to enlongate and the plant bends towards the light.