Which of the following structures in a plant is primarily responsible for regulating transpiration? a) Xylem b) Stomata c) Root hairs d) Phloem

The structure primarily responsible for regulating transpiration in plants is stomata. Stomata are tiny pores on the leaf surface flanked by guard cells. They control the opening and closing of these pores to balance gas exchange (CO₂ uptake for photosynthesis) and water vapor loss (transpiration). When stomata close, transpiration decreases, conserving water during drought or high temperatures. While xylem transports water, it does not regulate its loss. Root hairs absorb water but do not control its release, and phloem transports sugars, unrelated to transpiration regulation. Thus, stomata are the correct answer.