This question is checking basic knowledge of plant hormone behaviour under water stress, comparing the four major growth hormones against the one true stress hormone.
- Auxins: These drive cell elongation and directional growth responses. Their activity is not the plant's drought alarm signal, and auxin driven growth is usually held back, not boosted, when water is scarce.
- Gibberellic acid: This hormone pushes stem elongation and germination forward. Under drought, overall growth slows down, so gibberellic acid levels tend to drop rather than rise.
- Abscisic acid: This is the plant's dedicated stress hormone. The moment water potential in the plant falls, roots and leaves step up abscisic acid production, and this rising level closes the stomata to limit further water loss, exactly the response a drought calls for.
- Cytokinin: This hormone supports cell division and keeps leaves green longer. Drought stress lowers cytokinin supply from the roots, which is one reason leaves senesce faster during water shortage.
Since abscisic acid is the one hormone whose production is switched up specifically in response to water deficit, and its rise directly causes stomatal closure, it is the correct answer.
Let's summarize:
- Auxin, gibberellic acid and cytokinin activity all tend to fall, not rise, under drought stress.
- Abscisic acid is the hormone that rises under drought and drives stomatal closure.
So the endogenous hormone that increases under drought conditions is abscisic acid.