If \( q_f \) is the free charge on the capacitor plates and \( q_b \) is the bound charge on the dielectric slab of dielectric constant \( k \) placed between the capacitor plates, then bound charge \( q_b \) can be expressed as:
\( q_b = q_f \!\left( 1 - \frac{1}{k} \right) \)
\( q_b = q_f \!\left( 1 + \frac{1}{k} \right) \)
\( q_b = q_f \!\left( 1 + \frac{1}{\sqrt{k}} \right) \)
\( q_b = q_f \!\left( 1 - \frac{1}{\sqrt{k}} \right) \)