Correct Answer - C
Wherever a species donates a poton, it forms a conjugate base. Thus, to find the conjugate base of `OH^(-)` ion, we remove `H^(+)` and and put one more negative charge (i.e., decrease charge by 1):
`underset("Acid")(HO^(-))rarr H^(+) underset("Base")(+O^(2-))`