Carbon dioxide and oxygen enter and exit a leaf by diffusion. which structure (s) on a leaf allow (s) this process to happen?

The stomata and their guard cells allows this happen.

Stomata is like a hole or gap on a leaf, most of them are present in a bottom side of the leaf, since waxy cuticle is not present over there. And 2 guard cells make up a stomata.

Guard cells are able to control the size of the stomata, depending on the situation. For example, the guard cells close up during day time because a lot of sunlight may cause more water loss.

In conclusion, guard cells and the stomata are the main structures that allow carbon dioxide and oxygen (water too) diffuse in and out of leaves.