The structure of a plant that is responsible for transpiration is the stomata, which is option 3. While each option has a role in the overall functioning of a plant, only stomata specifically facilitate transpiration.

Option 1, xylem, is responsible for transporting water and nutrients from the roots to the rest of the plant. This is an essential function, but it is not directly related to transpiration.

Option 2, the root, is responsible for absorbing water and nutrients from the soil. While roots play a role in the water supply for transpiration, they do not directly cause the process to occur.

Option 4, the bark, provides protection and support for the plant. While it may contribute to the overall health and well-being of the plant, it is not involved in the process of transpiration.

Therefore, the correct answer is option 3, stomata. Stomata are small openings found on the surface of leaves and stems, and they allow for the exchange of gases, including the release of water vapor during transpiration.