**Core Concept**
The birth rate is a demographic measure used to express the number of live births per 1,000 people in the mid-year population of a given area. This indicator helps in understanding the fertility rate and population growth dynamics.

**Why the Correct Answer is Right**
The correct answer is **A. Live birth/1000 mid yr. population** because it accurately reflects the standard definition of birth rate. The mid-year population is used as the denominator to account for the dynamic nature of population growth. This ensures that the birth rate is representative of the population at a specific point in time. The World Health Organization (WHO) and other global health authorities use this metric to track and compare birth rates across different regions.

**Why Each Wrong Option is Incorrect**
**Option B:** This option is incorrect because it does not specify "live birth," which is a crucial component of the birth rate. A birth can be either live or stillbirth, but the birth rate specifically refers to live births.

**Option C:** This option is incorrect because it uses a denominator of 10,000 instead of the standard 1,000. This would significantly alter the birth rate calculation and make it less comparable to other regions.

**Option D:** This option is incorrect because it specifies a population of reproductive age (15-45) instead of the mid-year population. The birth rate should be based on the total population, not just the reproductive age group.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the birth rate is often expressed in conjunction with the death rate to calculate the crude birth rate (CBR) and crude death rate (CDR). These metrics provide valuable insights into the population's overall health and demographic trends.

**✓ Correct Answer: A. Live birth/1000 mid yr. population**