Answer:
[tex]\large\boxed{B.\ f(x)=4^x}[/tex]
Step-by-step explanation:
Exponential function [tex]f(x)=(a)^x[/tex] is
increasing if [tex]a > 1\to a\in(1,\ \infty)[/tex]
decreasing if [tex]0 < a < 1\to a\in(0,\ 1)[/tex]
[tex]A.\ f(x)=\left(\dfrac{1}{4}\right)^x\to a=\dfrac{1}{4}\in(0,\ 1)\to\text{decreasing}\\\\B.\ f(x)=4^x\to a=4\in(1,\ \infty)\to\text{increasing}\\\\C.\ f(x)=(0.4)^x\to a=0.4\in(0,\ 1)\to\text{decreasing}\\\\D.\ f(x)=\left(\dfrac{1}{2}\right)^x\to a=\dfrac{1}{2}\in(0,\ 1)\to\text{decreasing}[/tex]
