Most of the photosynthesis in a plant takes place in its leaves.
Leaves are broad and thin, giving them a large surface area for capturing sunlight. Tiny pores called stomata allow carbon dioxide to enter, while oxygen produced during photosynthesis exits. Veins bring water from the roots and carry sugars made in the leaf to other parts of the plant.
Inside the leaf, photosynthesis occurs in chloroplasts, especially in mesophyll cells. Chlorophyll within these chloroplasts absorbs light energy, which the plant uses to convert water and carbon dioxide into sugars, releasing oxygen as a by-product.
The answer is not exclusively “leaves”: green stems, young fruit, and other tissues can also photosynthesize. Cacti and some other plants rely heavily on green stems because their leaves are reduced to spines. Even so, for most familiar plants, the leaf is the canonical answer.