Mortality can be defined as -

The correct answer is Option (2) → The number of deaths in the population during a given period

The density of a population changes through four basic processes. Natality and immigration increase the density. Mortality and emigration decrease it.

(1) The number of births during a given period in the population — Incorrect. This is natality, which adds to the population.

(2) The number of deaths in the population during a given period — Correct. Mortality is the number of deaths in the population during a given period. It reduces the population density.

(3) The number of individuals of the same species that have come into the habitat from elsewhere during given time period — Incorrect. This is immigration, which adds individuals.

(4) The number of individuals of the population who left the habitat and gone elsewhere during given time period — Incorrect. This is emigration. The individuals are not dead. They have only moved away.

NCERT Extract:
Population Growth – “… (i) Natality refers to the number of births during a given period in the population that are added to the initial density.
(ii) Mortality is the number of deaths in the population during a given period.
(iii) Immigration is the number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration.
(iv) Emigration is the number of individuals of the population who left the habitat and gone elsewhere during the time period under consideration.”