Eliminate incorrect options: Observing the color change of leaves is not a reliable indicator of photosynthesis rate, as leaf color can change due to other factors like nutrient deficiencies or aging. Measuring the decrease in soil nutrients is unrelated to photosynthesis, as photosynthesis primarily involves light, water, and carbon dioxide. Counting the number of chloroplasts in a leaf cell does not measure the rate of photosynthesis but rather provides structural information.