The density of a population in a given habitat during a given time pe…

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

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

(A) Mutualism — Incorrect. It is an interaction in which two species both gain. It is not one of the four basic processes.

(B) Natality — Correct. It is the number of births in a given period. These add to the initial density.

(C) Emigration — Correct. It is the number of individuals of the population who leave the habitat in the given period. This lowers the density.

(D) Commensalism — Incorrect. It is an interaction in which one species gains and the other is neither harmed nor helped. It is not one of the four basic processes.

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.”