Answer: both options A and D
Explanation:
Half filled and completely filled orbitals are more stable than any other configuration since they are more symmetrical and energy exchange Occurs readily.
So 4f^7& 4f^14 are more stable
PH is the logarithmic measure of the concentration of hydrogen ions in solution. In an aqueous system, the lowest possible concentration of H+ ions (least acidic) is 1x 10^-14. The -log(1x10^-14) = pH of 14