If a dielectric slab is introduced between the plates of an isolated charged parallel plate capacitor

force of attraction between the plates remains same

The correct answer is option 3: force of attraction between the plates remains same

When the capacitor is isolated, the charge on the plates does not change. Now, even though inserting a dielectric increases the capacitance and reduces the potential difference and net electric field between the plates, the force between the plates depends on something more specific.

The force on one plate is due to the electric field produced by the other plate only, not the total field inside the capacitor. The field produced by a single plate is given by $\frac{\sigma}{2\epsilon_0}$​, which depends only on the charge density and does not change when a dielectric is inserted.

Since the charge remains the same and the field due to each plate remains unchanged, the force of attraction between the plates also remains unchanged.