In his experiments with grass seedlings, darwin covered the tips of some seedlings with transparent sheaths to see if.. options: plant naturally grew toward a source of light. auxins were produced in the tips of stems. the presence of a sheath affected plant growth. or auxins could be produced in the absence of light.

The correct option is this: PLANTS NATURALLY GROW TOWARD A SOURCE OF LIGHT.
Darwin carried out an experiment in which he found that when the tip of a plant is covered with an opaque material, the plant grow up straight but when it is covered with a transparent material that allow light, the plant bend toward the source of light. He thus, concluded that the tip of the plant is responsible for sensing light which is needed for photosynthesis.

Ithink the answer is d.