Assertion:. It is necessary to understand the ratio between the numbers of people to the size of land. This ratio is the density of population. 

Reasoning: Each unit of land has limited capacity to support people living on it. 

Both Assertion (A) and reasoning (R) are correct and R is the correct explanation of A.

The correct answer is Option 1: Both Assertion (A) and reasoning (R) are correct and R is the correct explanation of A.

Assertion:. It is necessary to understand the ratio between the numbers of people to the size of land. This ratio is the density of population. This statement is correct because population density refers to the relationship between the number of people and the area of land occupied by them.

Reasoning: Each unit of land has limited capacity to support people living on it. This statement is also correct because land and resources are limited and can support only a certain population.

Why Reason (R) correctly explains Assertion (A): Since every unit of land has a limited carrying capacity, it becomes important to study how many people are living on a particular area of land. This need leads to the concept of population density. Therefore, the Reason directly explains the Assertion.

NCERT: Each unit of land has limited capacity to support people living on it. Hence, it is necessary to understand the ratio between the numbers of people to the size of land. This ratio is the density of population. It is usually measured in persons per sq km.