Which of the following will have the highest pK_b value?

C₆H₅NH₂

The groups -CH₃, -OCH₃ and -NH₂ are all electron donating groups. When these are substituted at para position in aniline, it stabilizes the respective anilinium ion formed and thus increases the basic strength. Therefore, aniline is the weakest base from the given compounds and will have the highest pK_b value