Population density can be measured in terms of -

The correct answer is Option (2) → Biomass

Population density (N) is the size of a population. It is usually counted as the total number of individuals, but it need not be measured in numbers only. Where numbers give a wrong picture, per cent cover or biomass is a better measure. For example, one huge banyan tree plays a far bigger role in a community than 200 carrot grass plants, even though its number is only one.

(1) Volume — Incorrect. Volume is the space an object takes up. It is not used to express population size.

(2) Biomass — Correct. Biomass is the total mass of the living organisms. It is a meaningful measure of population size where counting gives a wrong picture.

(3) Mutation — Incorrect. A mutation is a sudden change in the genetic material. It is not a unit of measurement.

(4) Gene flow — Incorrect. Gene flow is the movement of genes from one population to another through migration. It changes allele frequencies; it does not measure population size.

NCERT Extract:
Population Attributes – “… In an area, if there are 200 carrot grass (Parthenium hysterophorus) plants but only a single huge banyan tree with a large canopy, stating that the population density of banyan is low relative to that of carrot grass amounts to underestimating the enormous role of the Banyan in that community. In such cases, the per cent cover or biomass is a more meaningful measure of the population size.”