**Core Concept**
The demographic transition model describes the changes in population growth patterns that occur as a country develops from a pre-industrial, agrarian society to an industrialized, urbanized society. The model consists of four phases: early expanding, late expanding, stationary, and post-stationary. The late expanding phase is characterized by a decline in mortality rates due to improvements in healthcare and sanitation, leading to a significant increase in population growth.

**Why the Correct Answer is Right**
During the late expanding phase, the population growth rate accelerates due to the decline in mortality rates, which is often accompanied by a slight increase in fertility rates. This phase is marked by a shift from a predominantly rural, agrarian economy to a more industrialized and urbanized economy, with a growing middle class and improved living standards. The increased availability of education, healthcare, and other social services contributes to the decline in mortality rates, particularly among children and infants.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the early expanding phase is characterized by high mortality rates and high fertility rates, leading to a slow population growth rate, whereas the late expanding phase is marked by declining mortality rates and increasing population growth.
* **Option B:** This option is incorrect because the stationary phase is characterized by a balance between birth and death rates, resulting in a stable population size, whereas the late expanding phase is marked by a significant increase in population growth.
* **Option C:** This option is incorrect because the post-stationary phase is characterized by a decline in fertility rates and an increase in mortality rates, leading to a decline in population growth, whereas the late expanding phase is marked by accelerating population growth.

**Clinical Pearl / High-Yield Fact**
The late expanding phase of the demographic transition model is often associated with significant social and economic changes, including urbanization, industrialization, and improvements in healthcare and education. Understanding the characteristics of this phase is essential for policymakers and public health officials to develop effective strategies for managing population growth and improving population health.

**Correct Answer: C. This phase is characterized by accelerating population growth due to declining mortality rates and slightly increasing fertility rates.**