An element A of group 1 shows similarity to an element B belonging...

Hydration enthalpy is inversely related to the size of the ion.

${Li}^{+}$ has maximum hydration enthalpy in group $1$ due to its small size.

Lithium shows diagonal relationship to magnesium.

The similarity between lithium and magnesium is particularly striking and arises because of their similar sizes : atomic radii, Li = 152 pm, Mg = 160 pm; ionic radii : Li⁺ = 76 pm, Mg²⁺= 72 pm.

The main points of similarity are

(I)Both lithium and magnesium are harder and lighter (ii) Lithium and magnesium react slowly with water. Their oxides and hydroxides are much less soluble and their hydroxides decompose on heating. Both form a nitride, Li3N and Mg3N₂ , by direct combination with nitrogen. (iii) The oxides, Li₂O and MgO do not combine with excess oxygen to give any superoxide. (iv) The carbonates of lithium and magnesium decompose easily on heating to form the oxides and CO₂ .

Solid hydrogencarbonates are not formed by lithium and magnesium. (v) Both LiCl and MgCl₂ are soluble in ethanol. (vi) Both LiCl and MgCl₂ are deliquescent and crystallise from aqueous solution as hydrates, LiCl·2H₂O and MgCl₂ ·6H₂O.

So '$B$' is $Mg$.