The density of a population in a given habitat during a given period,…

The correct answer is Option (3) → (B), (C) and (D) only

The density of a population is the number of individuals per unit area. It changes through four processes. Natality (births) and immigration (individuals coming in) raise it. Mortality (deaths) and emigration (individuals going out) lower it.

(A) Symbiosis — Incorrect. It is a close living together of two species. It is an interaction between species. It is not one of the four processes that add or remove individuals.

(B) Mortality — Correct. Deaths reduce the number.

(C) Natality — Correct. Births add to the number.

(D) Emigration — Correct. Individuals leaving the habitat reduce the number.

Note: A similar question was also asked in CUET (UG) 2025 on 29-05-2025 (Slot 1) and on 20-05-2025 (Slot 2).

NCERT Extract:
Population Growth – “The size of a population for any species is not a static parameter. It keeps changing with time, depending on various factors including food availability, predation pressure and adverse weather. In fact, it is these changes in population density that give us some idea of what is happening to the population – whether it is flourishing or declining. Whatever might be the ultimate reasons, the density of a population in a given habitat during a given period, fluctuates due to changes in four basic processes, two of which (natality and immigration) contribute to an increase in population density and two (mortality and emigration) to a decrease.”