Assertion (A): Sodium metal is stored in kerosene
Reason (R): The density of sodium is less than water
A and R are true but the R is not the correct explanation of A
Sodium is stored under kerosene because it does not react with kerosene. On the other hand, Sodium violently reacts with water.
Taken one by one - both the assertion and reason are true but the reason does not explain the assertion.