Explanation:

- Boron: This micronutrient is vital for cell wall formation and increases the stability and function of membrane structures, indirectly helping in water absorption.

- Potassium: It plays a crucial role in opening and closing stomata, which is essential for water regulation in plants.

- Iron: Mainly involved in chlorophyll synthesis and electron transport, not directly related to water absorption.

- Manganese: Primarily assists in photosynthesis and nitrogen assimilation, rather than direct water absorption.

Potassium is the correct answer, as it directly affects stomatal function and water uptake efficiency.