The correct option is A Both assertion and reason are true and reason is the correct explanation of the assertion
Population size is technically called population density. It is generally represented as ‘N’.
The most appropriate measure of population density is generally in terms of number.
Though population density is generally calculated in terms of number, sometimes it is meaningless.
For example, in a particular area there are 200 carrot grass plants, but only a single Banyan tree with a large canopy. If population density in terms of number is considered, then the density of carrot grass plants is certainly higher than the Banyan tree, but the ecological role of the Banyan tree in that area cannot be underestimated.
Therefore, measuring the number of individuals may not always be apt. In this case, the parameter biomass is more meaningful. The biomass of a single Banyan tree is far more than the 200 carrot grass plants.
Hence, both assertion and reason are true and reason is the correct explanation of the assertion.