Which of the following species can act as the strongest base?

Due to the higher electronegativity of sp2 hybridized carbon of phenol to which –OH is attached, electron density decreases on oxygen.

This increases the polarity of the O–H bond and results in an increase in ionization of phenols than that of alcohols and water.

when a group like -NO₂ is on the phenol the electron density of oxygen decreases more due to -R effect. Hence its tendency to attract the H⁺ increases more.

Thus, option (iv) is the strongest base (tendency to attract H⁺ ion).