The question provides us with an assertion (A): Sodium metal is stored under kerosene, and a reason (R): Metallic sodium melts when exposed to air.

Option 1 suggests that both the assertion and reason are true, and that the reason correctly explains the assertion. Option 2 also accepts A and R as true but denies that R explains A.

The correct answer, however, is Option 3: A is true, but R is false. This suggests that while it`s correct that Sodium metal is stored under kerosene, the reason provided is inaccurate. Sodium metal doesn`t melt when exposed to air, it reacts violently and can ignite.

Therefore, the assertion is accurate because Sodium is indeed stored under kerosene to prevent it from reacting with air (specifically the oxygen in the air) and potentially causing a fire or explosion, not because it melts when exposed to air.

Option 4 suggests that the assertion is false but the reason is true, which is incorrect.