## Core Concept
Life expectancy refers to the average number of years a person is expected to live based on age-specific mortality rates. It is a crucial indicator of the overall health and well-being of a population. Life expectancy at birth is a commonly used metric that reflects the mortality conditions prevailing in a particular year.

## Why the Correct Answer is Right
The correct answer, , indicates that according to the 2015 estimate, the life expectancy at birth is 71.6 years for the global population, though exact numbers can vary slightly based on the source (e.g., WHO, World Bank). This estimate considers various factors, including improvements in healthcare, sanitation, and reductions in mortality rates, particularly among children and in regions with previously high mortality rates.

## Why Each Wrong Option is Incorrect
- **Option A:** . This option suggests a lower life expectancy than the correct answer. Without specific numbers, it's hard to assess its accuracy directly, but given that the correct answer is , any option significantly lower would not align with global trends indicating an increase in life expectancy over the years.
- **Option B:** . Similar to Option A, if this suggests a life expectancy that is significantly different from , it would not align with known data trends.
- **Option C:** . This option seems to suggest a very low life expectancy and does not align with global estimates.

## Clinical Pearl / High-Yield Fact
A key point to remember is that life expectancy has been steadily increasing globally, primarily due to decreases in infant and child mortality rates, and improvements in healthcare for adults. However, disparities exist between countries and within populations due to factors like income inequality, access to healthcare, and lifestyle.

## Correct Answer: D. 71.6