B. Stomata
Stomata are tiny pores present on the surface of leaves through which massive amounts of gaseous exchange takes place.
The source passage (Chapter 5, Section 5.2.1) explicitly states that "Massive amounts of gaseous exchange takes place in the leaves through these pores (stomata)." Xylem transports water, phloem transports food, and the cuticle prevents water loss — none are responsible for gaseous exchange.