Final answer:
Water stress is more likely in (Option C) naturally arid ecosystems, heavily populated countries,
Step-by-step explanation:
Water stress occurs when the demand for water exceeds the supply, a condition that is likely to be observed in (Option C) naturally arid ecosystems, heavily populated countries, and areas with intensive, highly-irrigated agriculture. These conditions often coincide with the mismanagement of water resources, leading to a higher frequency of droughts, and reduced access to clean water. Factors such as deforestation exacerbate these problems by reducing natural water vapor contributions to the atmosphere. The emphasis on sustainable, holistic solutions including proper sewage treatment and sustainable agricultural practices is crucial for addressing water shortages and the ongoing global water crisis.