
A $4:2$ encoder gives the binary code of the active input.
If $\mathrm{I}_0=1$, output is $00$.
If $\mathrm{I}_1=1$, output is $01$.
If $\mathrm{I}_2=1$, output is $10$.
If $\mathrm{I}_3=1$, output is $11$.
Now check $\mathrm{Y}_1$.
$\mathrm{Y}_1=1$ for inputs $\mathrm{I}_2$ and $\mathrm{I}_3$.
So, $\mathrm{Y}_1=\mathrm{I}_2+\mathrm{I}_3$.
Now check $\mathrm{Y}_0$.
$\mathrm{Y}_0=1$ for inputs $\mathrm{I}_1$ and $\mathrm{I}_3$.
So, $\mathrm{Y}_0=\mathrm{I}_1+\mathrm{I}_3$.
Answer: B