To check the basicity of an ion, the conjugate acid must be weak. The conjugate acids are given below: $H _{2} O , CH _{3} OH , CH _{3} COOH$ and $HCl$.
We know that methanol is a stronger acid than water. Weaker the acid, stronger is the conjugate base, by that logic, $OH ^{-}$ is the strongest base among the given options.