Final answer:
Charles Darwin would be the scientist most concerned with the rate of photosynthesis, due to his work in evolution and natural selection.
Step-by-step explanation:
The scientist more likely to be concerned with the rate of photosynthesis in leaves is Charles Darwin. Albert Einstein is known for his work in physics, including the theory of relativity. Marie Curie is famous for her discoveries in radioactivity. Gregor Mendel is recognized for his work in genetics and heredity. However, Charles Darwin, known for his contributions to the field of evolution, would be most directly concerned with biological processes such as photosynthesis in the context of natural selection and adaptation of species.