$A\frac{?}{\frac{?}{?}}=\{a,b,c,d\}$

$x=\{1,2,3,4\}$

The function $f$:$Ax$ is defined as

$f=\{(a,4),(b,1),(c,4),(d,4)\}$

Select the set corresponding to the range of $f.$

$\{1\}$

$\{1,4\}$

$\{1,2,3,4\}$