Since I don't have access to the graph you mentioned, I cannot directly determine which letter corresponds to the greatest rate of photosynthesis as a function of light intensity. Generally, in a graph of photosynthesis against light intensity, you would look for the steepest slope or the highest point in the curve, which would indicate the greatest rate of photosynthesis.
If you can provide a description of the graph or the data it contains, I'd be happy to help analyze it!