What layer of a leaf contains chloroplasts for photosynthesis?

The palisade mesophyll is the correct choice because it is the layer within the leaf that is primarily responsible for photosynthesis due to its high concentration of chloroplasts. These chloroplasts contain chlorophyll, the green pigment that captures light energy from the sun, which is essential for the photosynthesis process. The palisade mesophyll is typically located just beneath the upper epidermis and is structured to maximize light absorption, often arranged in a columnar formation to optimize exposure to sunlight.

In contrast, the spongy mesophyll, while also containing some chloroplasts, is more involved in gas exchange and not as densely packed with chloroplasts as the palisade mesophyll. The epidermis serves as a protective outer layer and does not contain chloroplasts; its primary function is to prevent water loss and protect the internal structures of the leaf. The cuticle is a waxy layer that also helps reduce water loss, but it does not participate in photosynthesis since it contains no chloroplasts or living cells. Therefore, the palisade mesophyll is the definitive layer for photosynthesis in leaves.