The correct answer is Option (3) → $C_6H_5-NH_2$
Aniline is the least basic amine in the gaseous phase among the given options.
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In aniline (C₆H₅NH₂), the lone pair on nitrogen is delocalised into the benzene ring due to resonance.
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Because of this delocalisation, the lone pair is less available to accept a proton, which reduces its basic strength.
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Alkyl amines like (CH₃)₂NH, CH₃CH₂NH₂, and (CH₃)₃N have +I (electron-donating) effect, which increases electron density on nitrogen and makes them more basic.
Thus, due to resonance stabilisation and reduced availability of the lone pair, aniline is the least basic.
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