(a) Na and K

(b) K and C

(c) Na and S

(d) K and P

Solution:

Option (a) is the answer. Phenolphthalein is a synthetic indicator that remains colourless in acidic and neutral environments but turns pink in basic environments.

Because metal oxides are basic in nature, adding phenolphthalein causes the solution to become pink, indicating that it is a base.

Because Na and K are alkali metals, their oxides are basic, whereas C, S, and P are non-metals, and their oxides are acidic.

As a result, option (A) is the proper response.