**Core Concept**
The Age pyramid, also known as the population pyramid, is a graphical representation of a country's population distribution by age and sex. The spindle-shaped Age pyramid indicates a demographic transition, where the population is growing rapidly, with a large proportion of young people.

**Why the Correct Answer is Right**
A spindle-shaped Age pyramid is characterized by a wide base, representing the large number of children and young adults, and a narrower top, indicating a smaller proportion of older adults. This shape is typical of countries in the early stages of demographic transition, where high birth rates and low mortality rates result in a rapidly expanding population. The large number of young people in the population indicates a high reproductive potential, which can lead to further population growth.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a spindle-shaped Age pyramid does not indicate a stable or declining population.
**Option B:** This option is incorrect because a reverse pyramid does not have a wide base, but rather a narrow base and a wide top, indicating a low proportion of young people.
**Option C:** This option is incorrect because an hourglass-shaped Age pyramid does not have a wide base, but rather a narrow base and a wide top, indicating a low proportion of young people.

**Clinical Pearl / High-Yield Fact**
A demographic transition is a significant event in the history of a country, with far-reaching consequences for healthcare, education, and economic development.

**Correct Answer:** C.