Explanation:

- Option 1: Morbidity

- Morbidity refers to the condition of being diseased or the incidence of illness in a population.

- It indicates the prevalence of diseases that contribute to poor health.

- Morbidity is the correct answer for proneness to fall ill.

- Option 2: Mortality

- Mortality refers to the incidence of death within a population.

- It is used to measure the frequency of occurrence of death.

- Option 3: Both (a) and (b)

- This option suggests that both morbidity and mortality refer to proneness to illness, which is incorrect.

- Only morbidity addresses illness, while mortality addresses death.

- Option 4: None of the above

- This option implies that neither morbidity nor mortality is correct, which is inaccurate as morbidity correctly defines proneness to illness.