The relative nucleophilicity of ${\mathrm{NH}}_3, {\mathrm{H}}_2\mathrm{O}$ and $\mathrm{HF}$ towards bromoethane is

The relative nucleophilicity of ${\mathrm{NH}}_3, {\mathrm{H}}_2\mathrm{O}$ and $\mathrm{HF}$ towards bromoethane is ${\mathrm{NH}}_3>{\mathrm{H}}_2\mathrm{O}>\mathrm{HF}$.

Anions like${\mathrm{Cl}}^{-}$ or substances with a single pair of electrons like ${\mathrm{NH}}_3$ (ammonia), ${\mathrm{PR}}_3$, are examples of nucleophiles. In the example below, the oxygen of the hydroxide ion gives an electron pair to the carbon at the end of the bromopropane molecule to create a new chemical bond.

Hence, option B is correct.