**Core Concept**
Crude birth rate (CBR) is a demographic measure that calculates the number of live births per 1,000 people in a population per year, providing a general overview of fertility and reproductive health in a given area. It is an essential indicator in epidemiology and public health.

**Why the Correct Answer is Right**
The crude birth rate denominator is the mid-year population of the area being studied. This is because the CBR is calculated by dividing the total number of live births in a year by the mid-year population, and then multiplying by 1,000 to obtain the rate per 1,000 people. The mid-year population is used to ensure that the population is estimated at the middle of the year, thereby providing a more accurate reflection of the population at the time of births.

**Why Each Wrong Option is Incorrect**
**Option A:** The total population at the end of the year is incorrect because it would result in a biased estimate of the CBR, as the population would be inflated by births that occurred in the last few months of the year.
**Option B:** The total number of live births in the population is incorrect because it would result in a numerator, not a denominator.
**Option C:** The average annual population is incorrect because it would not accurately reflect the population at the time of births, which is the mid-year population.

**Clinical Pearl / High-Yield Fact**
When calculating the crude birth rate, it is essential to use the mid-year population as the denominator to ensure accuracy. This is because the mid-year population provides a more representative estimate of the population at the time of births, which is crucial for public health planning and policy-making.

**Correct Answer: C. Mid-year population.