The correct answer is A. Stomata.
Stomata are small pores found on the underside of leaves. They allow for the exchange of gases between the plant and the atmosphere. Carbon dioxide enters the plant through the stomata, and oxygen and water vapor exit the plant through the stomata.
Palisade tissue is a type of plant tissue that is found in the upper layer of leaves. It is made up of cells that are packed closely together and contain a lot of chlorophyll. Chlorophyll is a green pigment that absorbs sunlight and uses it to produce food for the plant.
Spongy tissue is a type of plant tissue that is found in the lower layer of leaves. It is made up of cells that are loosely packed together and contain air spaces. These air spaces allow for gas exchange between the plant and the atmosphere.
The midrib is the central vein of a leaf. It is made up of xylem and phloem, which are the two types of vascular tissue that transport water and nutrients throughout the plant.
In conclusion, the correct answer is A. Stomata. Stomata are small pores found on the underside of leaves that allow for the exchange of gases between the plant and the atmosphere.