Fluctuations in population density of a population in a given habitat…

The correct answer is Option (4) → All of these

The density of a population in a habitat changes all the time. These changes come from four basic processes. Two of them, natality (births) and immigration, add individuals; the other two, mortality (deaths) and emigration, remove them. A change in any of the four makes the density go up or down.

Option (1) Natality and immigration — Correct statement, but it is only part of the answer. Births and immigrants raise the density.

Option (2) Mortality — Correct statement, but it is only part of the answer. Deaths lower the density.

Option (3) Emigration — Correct statement, but it is only part of the answer. Individuals leaving the habitat lower the density.

Since all four processes together decide the fluctuation, the answer is Option (4).

NCERT Extract:
Population Growth – “… 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.”