Which of the following is the weakest Bronsted base?




4. CH3NH2

Hint: Resonance makes amine less basic 
Aniline is weakest Bronsted base among the given four compounds. In aniline, nitrogen lone pair participate in resonance with benzene ring due to which availability of an electrons for an acid decreases. 
The resonating structure of aniline is as follows:

Hence, lone pairs of nitrogen are less available for donation to the acid in aniline, thus, it is a weak base.