The action spectrum of photosynthesis was described in 1883 by:

Explanation

The correct answer is T.W. Engelmann. In 1883, Engelmann conducted experiments using a prism to disperse light across a filament of algae and used oxygen-sensitive bacteria to determine where photosynthesis was most active. This led to the first description of the action spectrum of photosynthesis, highlighting the effectiveness of different wavelengths of light. The other options involve significant figures in photosynthesis research but are associated with different discoveries: Robert Hill with the Hill reaction, C. Calvin with the Calvin cycle, and Hatch and Slack with the C4 pathway.