If $f(x)=4^x$ then $f(x+1)-f(x)$ equals:
$\text{(A)}\ 4\qquad\text{(B)}\ f(x)\qquad\text{(C)}\ 2f(x)\qquad\text{(D)}\ 3f(x)\qquad\text{(E)}\ 4f(x)$
The desired expression is equal to $4^{x+1} - 4^{x}$
Using the fact that $4^{x+1}$=$4^{x}*4$, we see that the answer is
$3*4^{x}$
$\fbox{D}$