Which of the following has the highest pKb value?

The correct answer is Option (3) → C₆H₅NH₂

The highest pK_b belongs to the weakest base. An electron releasing group on the ring raises the electron density on nitrogen and makes the base stronger. So plain aniline (pK_b 9.38) is the weakest base here.

Why the other options are incorrect

Option (1) p-CH₃OC₆H₄NH₂ — Incorrect. The –OCH₃ group releases electrons, so p-methoxyaniline is a stronger base than aniline and has a lower pK_b.

Option (2) p-CH₃C₆H₄NH₂ — Incorrect. The –CH₃ group releases electrons, so p-methylaniline is also a stronger base than aniline, with a lower pK_b.

Option (4) C₆H₅NHCH₃ — Incorrect. N-Methylaniline has pK_b 9.30, lower than 9.38 for aniline, so it is a slightly stronger base.

NCERT (2026-27 edition):
9.6 Chemical Reactions – “… Larger the value of K_b or smaller the value of pK_b, stronger is the base. The pK_b values of few amines are given in Table 9.3.”

Table 9.3: pK_b Values of Amines in Aqueous Phase

“… In case of substituted aniline, it is observed that electron releasing groups like –OCH₃, –CH₃ increase basic strength whereas electron withdrawing groups like –NO₂, –SO₃H, –COOH, –X decrease it.”