**Core Concept**
The carrying capacity of a population refers to the maximum number of individuals that an environment can sustain indefinitely. This concept is crucial in understanding the dynamics of population growth and its impact on the environment. The carrying capacity is influenced by various factors, including the availability of resources such as food, water, and shelter.

**Why the Correct Answer is Right**
The limiting resource is the primary factor that determines the carrying capacity of a population. This is because the availability of resources directly affects the population's ability to grow and sustain itself. When resources become scarce, the population growth rate slows down, and eventually, the population size stabilizes at a level that is in equilibrium with the available resources. This equilibrium is known as the carrying capacity. In other words, the limiting resource acts as a bottleneck that regulates population growth.

**Why Each Wrong Option is Incorrect**
**Option A:** The population growth rate is a measure of how quickly a population is increasing in size, but it does not determine the carrying capacity.

**Option B:** The birth rate is a factor that influences population growth, but it is not the primary determinant of carrying capacity.

**Option C:** The death rate is also a factor that affects population growth, but it is not the primary factor that determines carrying capacity.

**Clinical Pearl / High-Yield Fact**
A key concept to remember is that the carrying capacity is a dynamic equilibrium that can change over time due to changes in resource availability, technological advancements, or other environmental factors.

**✓ Correct Answer: D. Limiting resource**