Question

What makes a stem bend towards sunlight

Answer 1

it is a photo tropic movement of the stem of the plant.
the auxine present on the tip of the root have a property to move against the direction of sunlight.
so it started accumulating opposite side of the stem .as it is a growth hormone so the opposite side where auxine has deposited started growing fast and the side where auxine has not deposited will remain short this unequal growth in the plant shoot leads them to bend towards light.

Answer 2

Under normal light condition auxins are spread out in the plant. But when sunlight various, auxin is broken down on the sunnier side of the stem. The higher concentration of auxin on the Shady side causes the plant cells on that side to grow more so it bends towards the light.