# Which one of the following is least able to behave as a Lewis base: CH3NH2, (CH3)2NH, (CH3)3N , or (CH3)3NH+?

Trimethylammonium ion, ${\left[{\left({H}_{3} C\right)}_{3} N H\right]}^{+}$, is clearly least able to function as a Lewis Base; the nitrogen has been quaternized and is no longer available.
By definition, a Lewis Base is an electron pair donor, whereas a Bronsted base is a proton acceptor. Methylamine, ${H}_{3} C N {H}_{2}$, dimethylamine, ${\left({H}_{3} C\right)}_{2} N H$, and trimethylamine, ${\left({H}_{3} C\right)}_{3} N$ are all Bronsted bases and Lewis bases by virtue of the lone electron pair on the nitrogen centre.