Trimethylamine is the strongest base among the methylamines in the gas phase, but in water it is weaker than $\mathrm{CH}{}_3\mathrm{NH}{}_2$ ($\mathrm{p}{K}_a$ of conjugate acids: 9.8 against 10.6). The reason is that in water

1. Athe $+I$ effect of methyl groups vanishes in water
2. Bwater protonates trimethylamine incompletely because it is a stronger acid
3. Ctrimethylamine is not soluble in water at all
4. Dthe $(\mathrm{CH}{}_3){}_3\mathrm{NH}{}^{+}$ cation has only one $\mathrm{N}-\mathrm{H}$ to hydrogen-bond, so it is poorly stabilised by solvation