Answer: B. Hydrotropism
The response that helps aerial parts of the plant to find water is called as hydrotropism. Tropism refers to the response of the plant growth towards a particular stimuli, such as gravity, sunlight, touch and water. The hydrotropism refers to the growth response of the plant parts towards water. The plant may show positive or negative hydrotropism. A positive hydrotropism refers to the response in which the plant grows towards water whereas the negative hydrotropism refers to a response in which the plant grows away from it.
In the given condition, the plants growing high in the tree canopy send out roots. The roots will absorb the water present in the humid air. Therefore, it is a positive hydrotropism response.