The correct answer is: b) Life expectancy at birth.
Life expectancy at birth is a measure of the average number of years that a newborn is expected to live if current mortality rates continue to apply. It is a good indicator of the overall health of a population, as it takes into account factors such as access to healthcare, nutrition, and sanitation.
Infant mortality rate is the number of deaths of infants under one year of age per 1,000 live births. It is a good indicator of the quality of healthcare in a country, as it reflects the number of deaths that are preventable with access to adequate medical care.
Prevalence of diseases is the number of people in a population who have a particular disease at a given time. It is a good indicator of the overall health of a population, as it reflects the burden of disease that people are living with.
Number of hospitals is not a good indicator of the health of a population, as it does not take into account factors such as the quality of care that is provided in hospitals or the accessibility of healthcare.
In conclusion, the health component of the HDI is typically measured by life expectancy at birth. This is because life expectancy at birth is a good indicator of the overall health of a population, as it takes into account factors such as access to healthcare, nutrition, and sanitation.