The treatment of alkali (NaOH) with beryllium hydroxide causes geometrical change of product from:

It is challenging to dissolve beryllium hydroxide in water. It dissolves in alkalis to produce the compound tetrahydroxoberyllate,  ${\left[\mathrm{Be}(\mathrm{OH}{)}_4\right]}^{2-}$ . As a result, the molecule's geometry changes from linear to tetrahedral. Hence option (b) is correct.