For a specie to be a strong base, the corresponding conjugate acid should be weak.$\mathrm{OH}^- + \mathrm{H}^+ \rightarrow\; \underset{\text{water}}{\mathrm{H_2O}}$$\mathrm{OR}^- + \mathrm{H}^+ \rightarrow\; \underset{\text{alcohol}}{\mathrm{ROH}}$$\mathrm{C_6H_5O}^- + \mathrm{H}^+ \rightarrow\; \underset{\text{phenol}}{\mathrm{C_6H_5OH}}$$\mathrm{C_6H_5O}^- (\mathrm{m-NO}_2) + \mathrm{H}^+ \rightarrow\; \underset{\text{m-nitrophenol}}{\mathrm{HOC_6H_5}(\mathrm{m-NO}_2)}$Since the acidity of phenol is higher than aliphatic alcohol. Also, the acidity of water is higher than alcohol. Thus least acidic product is ROH.As weaker acid has a strong conjugate base or vice-versa, $\mathrm{RO}^-$ will be the strongest base among all.