We know that the humidity and atmosphere pressure are inversely proportional to each other hence when the humidity of atmosphere falls below 50% the pressure is increased and the stomata is closed during that time due to increase pressure as we all know that these phenomena works on concentration gradient.
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Stomata Closure and Relative Humidity

Introduction:
Stomata are tiny openings found on the surface of leaves and stems of plants. They play a crucial role in the exchange of gases, allowing carbon dioxide to enter the plant for photosynthesis and releasing oxygen and water vapor back into the atmosphere. The opening and closing of stomata are regulated by various factors, including light intensity, temperature, and relative humidity.

Stomatal Opening and Closing:
Stomata open during the day to facilitate the uptake of carbon dioxide for photosynthesis. This opening is controlled by the turgor pressure of the guard cells that surround the stomatal pore. When the guard cells are turgid, the stomata open, and when the guard cells lose turgor, the stomata close.

Role of Relative Humidity:
Relative humidity refers to the amount of moisture present in the atmosphere compared to the maximum amount it can hold at a given temperature. The relative humidity affects the rate of transpiration, which is the process of water loss through the stomata.

Stomatal Closure and Relative Humidity:
When the relative humidity of the atmosphere falls below a certain threshold, stomata tend to close to reduce the rate of transpiration. The correct answer to the given question is option 'A' (50%) because it represents a relatively low relative humidity level.

Reasoning:
When the atmosphere becomes drier, the plant faces an increased risk of losing excessive amounts of water through transpiration. Closing the stomata helps to conserve water and maintain the internal water balance of the plant. This is particularly important during times of water scarcity or drought conditions.

Preventing Water Loss:
Closing the stomata prevents water loss by reducing the surface area available for transpiration. When the stomata are closed, the movement of water vapor from inside the leaf to the outside is significantly reduced. This helps to conserve water and prevent wilting, which can be detrimental to plant health.

Adaptation to Environmental Conditions:
Plants have evolved various mechanisms to adapt to different environmental conditions. Closing stomata during periods of low relative humidity is one such adaptation. By closing the stomata, plants can minimize water loss and survive in arid or dry environments.

Conclusion:
In conclusion, stomata close down when the relative humidity of the atmosphere falls below a certain threshold. This is an adaptive mechanism employed by plants to reduce water loss and maintain their internal water balance. The correct answer to the given question is option 'A' (50%).