An osmoregulatory adaptation in xerophytes is that their

Explanation

The correct answer is that xerophytes often close their stomata during the daytime to minimize water loss through transpiration, which is crucial for their survival in arid environments. This is a specific osmoregulatory adaptation. The other options are not directly related to osmoregulation in xerophytes: well-developed vascular tissues are structural adaptations for transport rather than water conservation, multicellular reproductive organs are a general plant feature, and salt glands are typical of halophytes, not xerophytes.