In which of the following compounds the weight percentage of ' $C$ ' and ' $H$ ' are 80 and 20 respectively?

In methane, ${\mathrm{CH}}_4$ we know, the weight $\%$ of carbon is $75\%·\left(\frac{1\times 12}{16}\times 100=75\%\right)$

considering ${\mathrm{C}}_2{\mathrm{H}}_6$,

The total molar mass of ${\mathrm{C}}_2{\mathrm{H}}_6=12\times 2+6\times 1=30$

Wt. % of C $=\frac{2\times 12}{30}\times 100=80\%$

And the weight $\%$ of $\mathrm{H}=100-\%\mathrm{C}=20\%$

So, the compound is ${\mathrm{C}}_2{\mathrm{H}}_6$.