Assertion: Sodium reacts vigorously with cold water but magnesium does not react with cold water.
Reason: Sodium is more reactive than magnesium.

The correct option is D Assertion: True, Reason: True

Reason is the correct explanation of assertion.
Sodium reacts vigorously with cold water to produce sodium hydroxide and hydrogen gas. A lot of heat is released in this reaction. Thus, the hydrogen gas evolved immediately catches fire. This can be represented as:

$\text{Sodium + Water}\to \text{Sodium hydroxide + Hydrogen gas}$

On the other hand, magnesium does not react with cold water and it doesn't even react with hot water. It reacts only with steam. Thus, we can say that sodium is more reactive than magnesium. Hence, both assertion and reason are correct and reason is the correct explanation of the assertion.