An element A of group 1 shows similarity to an element B belonging to group 2. If A has maximum hydration enthalpy in group 1, then B is

Since element A belongs to Group 1 and shows similarity to element B in Group 2, and given that A has the maximum hydration enthalpy in Group 1, we first identify the member of Group 1 with the highest hydration enthalpy.

In Group 1 (alkali metals: Li, Na, K, Rb, Cs), hydration enthalpy decreases down the group because the ionic size increases. Therefore, Lithium (Li), having the smallest ionic radius, exhibits the maximum hydration enthalpy.

Furthermore, elements in the periodic table often display diagonal relationships, whereby an element resembles the one diagonally below and to the right. Consequently, Lithium (Group 1, Period 2) shows a diagonal relationship with Magnesium (Group 2, Period 3).

This similarity between Li and Mg arises from their comparable charge densities (charge/size ratio), electronegativity values, and polarizing powers.

Hence, element B is Mg, and the correct answer is Option A: Mg.