Discussion
The amine with the highest pKb value is
Answer: C
The highest pKb belongs to benzenamine (aniline).
A higher pKb means a weaker base. In aniline the nitrogen lone pair is delocalised into the benzene ring, so it is much less available for protonation; aniline's pKb is about 9.38.
N-Methylaniline is slightly stronger (pKb about 9.30) because the methyl group donates electrons. The aliphatic amines are far stronger bases, with pKb near 3.3-3.4 for methanamine, ethanamine and N-methylmethanamine.
The order of base strength in water among the aliphatic amines (secondary > primary > tertiary for methyl groups) does not affect this question, because every aliphatic amine is far stronger than an aromatic one.
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