Which structure in a leaf is mainly responsible for gaseous exchange?

As stated in the chapter, 'stomata are tiny pores present on the surface of the leaves. Massive amounts of gaseous exchange takes place in the leaves through these pores for the purpose of photosynthesis.' The opening and closing of the stomatal pore is controlled by the swelling and shrinking of the guard cells on either side of it, which regulates the exchange of $\text{CO}_2$ and $\text{O}_2$ as well as water loss.

(A) Xylem is incorrect — xylem is a vascular tissue that transports water and minerals from roots to leaves; it plays no role in gaseous exchange.

(C) Phloem is incorrect — phloem transports the soluble products of photosynthesis (translocation); it is not involved in gas exchange.

(D) Cuticle is incorrect — the waxy cuticle covering the leaf surface restricts, rather than aids, gaseous and water exchange, helping to reduce uncontrolled water loss; it is the opposite of an exchange structure.

Hence, stomata (option B) are mainly responsible for gaseous exchange in a leaf, as described in the chapter.