Assertion: The planet Saturn would float if put in water.
Reason: Density of water is more than the effective density of Saturn.
(A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
(B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
(C) Assertion is correct but Reason is incorrect
(D) Assertion is incorrect but Reason is correct

Hint
The ability of an object to float on water is determined by its density and not its size. The planet Saturn is mostly made up of gases like hydrogen and helium.

Complete step by step answer
Our solar system consists of eight planets, with Mercury being the smallest and Jupiter being the largest planet. Next in line to Jupiter is Saturn: the ring planet. Saturn is composed primarily of atmospheric gases like hydrogen and helium unlike other planets like Mars and Earth that are made of rocks. Hence, it is also referred to as a gas giant. Saturn also has a very large spin which hikes up the wind speed at its surface to about 1800 kilometres per hour. Although made of gases, this planet also has a heat source at its centre. Coming to its density, we know density is calculated as the fraction of mass per unit volume. We can see that the density depends directly on mass and inversely on the volume. So because of Saturn’s gas composition, the mass is very low as compared to other giants. Adding to that, as Saturn is the second largest planet in the solar system, it has a huge volume. Both of these factors together contribute to a very low density. It is estimated that the density is in fact so low that Saturn would float on water. This is because we know that objects with a lighter density than water tend to float on it.
This concludes that both the Assertion and the Reason are correct, and the Reason also accurately explains the Assertion, Hence, the correct answer is option (A).

Note
The actual density of Saturn is $687kg/{m^3}$ while that of water is $1000kg/{m^3}$. This is the average specific density. The core of Saturn is actually denser than water because it is estimated to be made up of rocks and molten metals.