Correct answer: D. Fertile soil and irrigation facilities
Explanation: Population density tends to rise where people can obtain food, water, income, and services reliably. Fertile soil supports good agricultural production, while irrigation reduces dependence on uncertain rainfall. Together, these conditions can create regular farm work, increase food availability, support markets, and encourage roads, storage, schools, and other services. More people may then settle in the region or remain there permanently.
Option D is correct because fertile soil and irrigation directly strengthen the economic base of a rural area. Persistent waterlogging, inaccessible steep slopes, and extreme aridity usually restrict farming, transport, construction, or health, so options A, B, and C generally discourage dense settlement. Irrigation does not remove every environmental problem, but combined with fertile land it is a strong factor favouring higher population density.