Which of the following growth responses is specifically directed toward light?

Phototropism is the growth response that specifically directs plant growth towards light sources. This phenomenon occurs due to the unequal distribution of plant hormones known as auxins, which promote cell elongation. When light shines on a plant from one direction, auxins accumulate on the shaded side, causing those cells to elongate more than those on the light-exposed side. As a result, the plant bends toward the light source to maximize photosynthesis and energy capture.

The other growth responses listed involve different stimuli. Gravitropism relates to movement in response to gravity, influencing root growth downward and stem growth upward. Thigmotropism describes a plant's growth response to physical touch, commonly observed in climbing plants that wrap around supports. Hydrotropism involves a plant's response to moisture gradients in the soil, leading roots to grow toward areas with higher water availability. Therefore, none of these other responses pertain directly to light stimuli like phototropism does.