Three important plant experimentation discoveries by scientists within the last 300 years are:

They were as follows:
Nicholas de Saussure was the first chemist to begin measuring and looking at the process of photosynthesis.

Jan Baptista's experiments showed that the loss of soil mass was smaller than the gain of plant mass and that it was related to water.

Ingenhousz found that air purification would only happen when the sun's energy and green plants worked together in the system.

Hope this helps.

The answer is c) tons