Conjugate acid of $\mathrm{H}{\mathrm{F}}_2^{-}$ is

The increasing basicity order of the following compounds is:

Bond length of ethane $\left(\mathrm{I}\right),$ ethene $\left(\mathrm{II}\right),$ ethyne $\left(\mathrm{III}\right)$ and benzene $\left(\mathrm{IV}\right)$ follows the order:

Arrange the following amines in the decreasing order of basicity.