Final answer:
New leaves are most photosynthetically active because they usually have higher chlorophyll content and are more efficiently positioned to capture sunlight and perform photosynthesis compared to older leaves.
Step-by-step explanation:
The statement that new leaves are most photosynthetically active is True. Leaves are the primary sites of photosynthesis, the process by which plants convert light energy into chemical energy to synthesize food. New leaves, in particular, tend to have a higher chlorophyll content and are more efficient at photosynthesis compared to older leaves. The green pigment chlorophyll plays a critical role by capturing sunlight. New leaves are often more exposed and better positioned to absorb sunlight and carry out photosynthesis efficiently. Additionally, yellow-leaved seedlings that are unable to convert light energy to chemical energy would show stunted growth or death by day 21 since they cannot synthesize the food they need to grow.