The correct option is B Assertion: True, Reason: True
Reason is the correct explanation of assertion.
Sodium reacts vigorously with water to produce sodium hydroxide and hydrogen gas. The hydrogen gas evolved immediately catches fire. This can be represented as:
Sodium + Water→Sodium hydroxide + Hydrogen gas
On the other hand, magnesium does not react with cold water and hot water. It reacts only with steam. Hence, both assertion and reason are correct and reason is the correct explanation.