(d) II
The guard cells (region II) surround the stomatal pores, through which massive amounts of gaseous exchange takes place in leaves for the purpose of photosynthesis.
Source: Life Processes, Section 5.2.1
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The textbook states: "Massive amounts of gaseous exchange takes place in the leaves through these pores (stomata) for the purpose of photosynthesis." In a standard leaf cross-section diagram, label II typically points to the stomatal pore/guard cells region. Examiners expect you to link gaseous exchange directly to stomata, not to mesophyll or epidermal cells. Remember: guard cells control the opening and closing of stomatal pores.