A thin metallic spherical shell contains a charge Q on its surface. A point charge q₁ is placed at the centre of the shell and another charge q₂ is placed outside the shell. All the three charges are positive. Then, the force on charge q₁ is

Due to Electrostatic shielding, the metallic spherical shell acts as a shield and do not allow the electric field due to outside charges to penetrate through it. Hence, electric field inside shell due to Q and q₂ is zero.
Therefore, no force will act on charge q₁.