According to the graph below, at which point is the plant performing the most
photosynthesis?

a. point c
b. point a
c. point b
d. point d

Point C

Explanation:

Oxygen is a by-product of photosynthesis, so when the oxygen production is at its highest, photosynthesis is also at its highest.

A. Point C

Explanation:

The plant which is evolving most oxygen is performing the most photosynthesis because during photosynthesis inorganic carbon dioxide and water react in the presence of sunlight to synthesis organic molecule known as glucose and oxygen is evolved.  

The reaction for photosynthesis is depicted as under:

      6CO₂       +   6H₂O    +  sunlight      →    C₆H₁₂O₆    +     6O₂

   (carbon          (water)                                 (glucose )       (oxygen)

   dioxide)  

So, more evolution of O₂ (oxygen) implies more photosynthesis.