O^2- and Si⁴⁺ are isoelectronic ions. If the ionic radius of O^2- is. 1.4A^o, that of Si⁴⁺ is

O²⁻   and Si⁴⁺ have 10 electrons each. Thus they are isoelectronic. 

For isoelectronic species, atomic radius is inversely proportional to Z effective. Z effective is directly proportional to the positive charge.

More the positive charge more the Z effective. Si⁴⁺ has +4 charge so it will have a shorter radius than O²⁻

Hence required answer is 0.41A^o