Assertion:
 $Li$ and $Mg$ Elements are bridge elements
Reason:
Period 2 elements exhibit similar properties with period 3 elements. Hence they are called bridge elements.
A. Both assertion and reason are correct and reason is the correct explanation for assertion
B. Both assertion and reason are correct but reason is not the correct explanation for assertion
C. Assertion is correct but the reason is incorrect.
D. Both assertion and reason are incorrect.

Hint: The elements that occur in nature are classified in the periodic table based on their atomic number. The elements in the atomic table are categorized into blocks that make them easier to correlate and thus make the comparison easie.

Complete step by step answer:
The elements in the periodic table are classified in the form of blocks, groups, and periods based on the properties that they show. The change in the group or the period shows the change in the number of shells and the number of electrons in the shell.
The elements in-between groups 3 through 12 are called the transition elements. These contain the elements that sown more of the metallic properties like iron, copper, etc. They are the only elements in the periodic table to produce the magnetic field.
The elements of the second period tend to show properties that are similar to the elements of the third period like lithium has properties that are similar to magnesium while the element beryllium shows properties that are close to the properties of aluminium.
Due to these properties of the similarity of the nature and reactions of the second and the third-period elements that they are termed as bridge elements.
Since both the assertion and the reason are correct.

So, the correct answer is Option A.

Note: Lithium acts as a bridge element between the elements of group 1 and group 2. It shows similar properties with sodium as both of them are in the same group. It also shows similar properties with magnesium because of the similar electronegativity and thus they cancel out each other due to the ion polarization.