Final answer:
A characteristic of a country with a large, fast-growing population is increasing energy demands due to the need to support a larger, often younger population with resources for their wellbeing and economic activities.
Step-by-step explanation:
The characteristic of a country with a large, fast-growing population is b. increasing energy demands. In countries with rapidly growing populations, energy needs rise as more people require resources for everyday life and economic activities. These countries often face challenges in meeting these demands sustainably. Populations with younger age structures, typically found in less economically developed countries, necessitate more resources for healthcare, education, and other services, leading to increased energy consumption. In contrast, economically developed countries tend to have slower population growth rates and have higher gross domestic products (GDP), leading to higher GDP per capita, which is associated with a higher standard of living. An economically underdeveloped country usually has many very young individuals and few old, indicating it's still in an earlier stage of demographic transition towards lower growth rates and improved living standards.