Explanation:

- Option 1: Mortality

- Life expectancy at birth is directly influenced by mortality rates.

- A decrease in mortality rates, particularly infant mortality, increases average life expectancy.

- Option 2: Fertility

- Fertility rates are more about birth rates and are not directly reflected in life expectancy figures.

- Life expectancy focuses on survival rates, not birth rates.

- Option 3: Health of Population

- While life expectancy provides insights into the general health of a population, it is not solely determined by people's overall health.

- Option 4: Provision of Health Facilities

- Good healthcare facilities can increase life expectancy, but life expectancy is a broader measure that includes more factors.

- Correct Answer: Option 1, Mortality

- Life expectancy at birth is mainly a measure of mortality rates.