Thomas' Calculus 13th Edition

The function $f$ is more sensitive to a change in $d$.
$f (a,b,c,d)=ad-b c$ and $f_a=d \\ f_b=-c \\ f_c=-b \\ f_d=a$ $\implies df = d \ da -c \ db -b \ dc +a \ d \ d$ $\implies df=2 dx +1 dy$ So, we can see that as |a| is bigger than b, c and d, the function $f$ is more sensitive to a change in $d$.