Identify the correct statements : A. Effective capacitance of a...

A. Effective capacitance of series combination is always smaller than the smallest capacitance - TRUE. For series: $\frac{1}{C_{eff}} = \frac{1}{C_1} + \frac{1}{C_2} + \ldots$, therefore C_{eff} < C_\min
B. Dielectric prevents charge displacement - FALSE. A dielectric allows polarization (displacement of bound charges) but prevents free charge movement. It does not prevent displacement of bound charges.
C. Increasing area or decreasing thickness increases capacitance - TRUE. From $C = \frac{\varepsilon_0 \varepsilon_r A}{d}$, increasing $A$ or decreasing $d$ increases $C$.
D. Concentric spherical shells centered at a point charge location are equipotential surfaces - TRUE. For a point charge, the potential at distance $r$ is $V = \frac{kq}{r}$, which depends only on distance, making concentric spheres equipotential surfaces.
Correct statements: A, C, D