Explanation:

- Transpiration: The correct answer. Plants release water vapor into the air through small openings in their leaves called stomata. It helps in cooling the plant, maintaining nutrient flow, and contributing to the water cycle.

- Photoperiodism: This is the response of a plant to the length of daylight. It influences flowering and other growth processes, not water release.

- Photosynthesis: This is the process by which plants use sunlight to convert carbon dioxide and water into glucose and oxygen. It is essential for plant energy production but doesn't directly involve water release into the air.

- Tropism: This is the movement or growth of a plant in response to environmental stimuli like light (phototropism) or gravity (gravitropism). It does not relate to water release.