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

Let`s break down each part of the option to understand it better.

Assertion (A) states that it is easier to swim in sea-water compared to ordinary water. This means that swimming in sea-water requires less effort or is less tiring than swimming in ordinary water.

Reason (R) states that the density of sea-water is higher than that of ordinary water. Density is a measure of how much mass is packed into a given volume. If the density of a fluid is higher, that means there is more mass packed into the same volume, making it denser.

In this case, the option suggests that the higher density of sea-water is the correct explanation for why swimming in it is easier compared to ordinary water.

The option concludes that both A and R are true, and R explains A correctly. This means that the higher density of sea-water, which makes it denser than ordinary water, is the reason why swimming in sea-water is easier.

Therefore, the correct answer is indeed option 1.