Acidic nature depends upon nature of electron withdrawing group and electronegativity. Electronegativity further depends on % of s character. Higher the s-character, greater will be the electronegativity and hence tendency to loose H increases thus acidic character also increases.



sp- hybridisation (50% s character) (pK_a = 1.86)



sp² - hybridisation (33-33% s character) (pK_a = 4.3)



sp² - hybridisation (Resonance effect) (pK_a = 4.5)



sp³ - hybridisation (25% s-character) (pK_a = 4.8)

Hence, acidic order I > II > III > IV.

II is more acidic than III since electron donating group ($$ - OC{H_3}$$) is attached to benzene ring in III which decreases the acidic character.

On the other hand, pK_a value also determined acidic nature, lower pK_a value gives maximum acidic character.

Hence, option (b) is correct.