in the decoder, A$_1$ = C and A$_0$ = I$_0$ ( replace the Gates with OR gates, then you can easily observe the o/p )

==> F = A'$_1$.A'$_0$ + A'$_1$.A$_0$ = A'$_1$.(A'$_0$+A$_0$) = A'$_1$ = C'

==> F = A'$_1$.A'$_0$ + A'$_1$.A$_0$ = A'$_1$.(A'$_0$+A$_0$) = A'$_1$ = C'