Option 1 states that both the Assertion (A) and the Reason (R) are true, and that R is the correct explanation of A.

First, let`s examine the Assertion (A): "Physiological density is preferable to arithmetic density as an index of population density." This means that the physiological density is a better measure for population density than the arithmetic density.

Now let`s look at the Reason (R): "Physiological density is based on arable land while arithmetic density is based on total area." This is stating that the physiological density is calculated using the amount of arable land, while the arithmetic density is calculated using the total area.

In this case, both the Assertion and Reason are true. Physiological density indeed considers the availability of arable land, which is important for determining a region`s ability to sustain its population. On the other hand, arithmetic density only takes into account the total area, which may not accurately reflect the actual population density. Therefore, the Reason provides a valid explanation for why physiological density is preferable to arithmetic density.

Based on this analysis, Option 1 is the correct answer, as it correctly states that both A and R are true, and R is the correct explanation of A.