The correct answer is option 3, leaf margin. Stomata are microscopic pores found on the surface of leaves, stems, and other plant parts. They are responsible for gas exchange, allowing carbon dioxide to enter the plant and oxygen to exit during photosynthesis. The leaf margin refers to the edge of the leaf, where some stomata are present.

Option 1, veins, is incorrect because stomata are not usually found within the veins of a leaf. Option 2, ventral surface of the leaf, is also incorrect because stomata are typically more abundant on the upper surface of the leaf, called the adaxial surface. Option 4, dorsal surface of the leaf, is also incorrect because stomata are usually more abundant on the lower surface of the leaf, called the abaxial surface.