Who discovered the C4 pathway?

The correct option is Hatch and Slack.

The C₄ pathway, also known as the dicarboxylic acid pathway or the Hatch-Slack pathway, is an alternative metabolic pathway used by certain plants (like maize, sugarcane, and sorghum) to fix carbon dioxide during photosynthesis.
This pathway minimizes photorespiration and optimizes water-use efficiency in hot and dry environments.

The pathway was named after the Australian botanists Marshall Davidson Hatch and Roger Kenneth Slack, who fully elucidated and described the detailed steps of this carbon fixation pathway in the late 1960s (specifically in 1966). Therefore, the discovery of the C₄ pathway is credited to Hatch and Slack.