The reason for selecting option (3) "Morbidity" as the one that implies proneness to fall ill is as follows:

1. **Morbidity**: Morbidity is a term used to describe the prevalence or incidence of illness, injury, or disease within a population. It measures the extent to which individuals in a population are affected by health problems or are prone to falling ill. High morbidity rates indicate a higher likelihood of people getting sick or experiencing health issues. Therefore, morbidity is directly related to the state of health and proneness to illness in a population.

In contrast, the other options have different meanings:

2. **Mortality**: Mortality refers to the death rate within a population and is not directly related to illness or proneness to fall ill. It measures the number of deaths in a population, not the prevalence of illness.

3. **Fertility**: Fertility relates to the birth rate, indicating the number of births in a population, and is unrelated to illness or health status.

4. **Infant Mortality**: Infant mortality specifically refers to the death rate among infants in a population, which is related to health but focuses on the death of very young children, not overall illness or proneness to fall ill.

Therefore, "Morbidity" is the option that directly addresses the state of health and the likelihood of individuals falling ill or experiencing health issues within a population.