Answer:
a) stomata
Step-by-step explanation:
The stomata are little pores / doorways / openings that have the express purpose of allowing gases in and out. Without this, plants would not be able to fix carbon, which is essential in respiration.
Why not c or b: The xylem and phloem channel water through the plant, not gases.
Why not d: The cuticle is the waxy outer layer of the leaf meant to protect it, not allow gases through.