Assertion: Element with three shells is kept in the third period.
Reason: The number of shells in an element is equal to the period number in which it is kept.
The number of shells in the atom of an element indicates the period in which it will be kept. The number of shells of an element is equal to the period number in which the element is kept. Therefore, elements with three shells will be kept in the third period. So, the reason is the correct explanation for the assertion given to us.