${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Cl} + \mathrm{X}\to {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{F}$. 

In the above reaction, ‘X’ cannot be

In chemical processes, electronegativities—the chemical elements which are located in the lower left quadrant of the periodic table tend to lose electrons.

Mercury is a non-metal with a chemical formula $\mathrm{Hg}$. Fluorine is the most electronegative element in the periodic table, and mercury has a lower electronegativity than fluorine. Fluorine and mercury combine to generate several chemicals.

Therefore in the chemical reaction 'X' can not be  ${\mathrm{Hg}}_2{\mathrm{F}}_2$.