Transpiration occurs in leaf air spaces

Steps:

• Water travels from xylem vessels through mesophyll cells via osmosis and diffusion.
• In mesophyll cells, water reaches cell walls exposed to intercellular air spaces.
• Evaporation happens from wet cell walls into the air spaces due to concentration gradient.
• Water vapor then diffuses out through stomata.

Why C is correct:

• Evaporation is the change from liquid water on mesophyll cell walls to vapor in air spaces, as defined in transpiration process.

Why the others are wrong:

• A: Xylem transports water but does not evaporate it.
• B: Mesophyll cells absorb water but evaporation occurs at their surfaces into air spaces.
• D: Stomata allow vapor exit but evaporation precedes this in internal spaces.

Final answer: C