In the gaseous state, the correct basic strength among the following is

 In gaseous phase, basic strength decreases in the order: ${\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_3 \mathrm{N}>{\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_2\mathrm{NH}>{\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{NH}}_2>{\mathrm{NH}}_3$. The $+\mathrm{l}$ effect increases with an increase in the alkyl group. Therefore, the basic strength will be the highest in ${\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_3 \mathrm{N}$ and the lowest in ${\mathrm{NH}}_3$. In aromatic compounds, the basicity increases as the electron-withdrawing group decreases and the electron-donating group increases.

Hence, the correct answer is option A) ${\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_3 \mathrm{N}>{\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_2\mathrm{NH}>{\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{NH}}_2>{\mathrm{NH}}_3$.