Answer: [tex]\bold{D.\ \bigg(-\infty, \dfrac{ln\ 4}{0.2}\bigg)}[/tex]
Step-by-step explanation:
[tex]y=\dfrac{c}{1+ae^{-rx}}\qquad \text{where growth rate increases from}\ -\infty\ to\ \dfrac{ln\ a}{r}[/tex]
[tex]f(x) = \dfrac{15}{1+4e^{-0.2x}}\qquad \rightarrow \qquad a=4, r=0.2[/tex]
[tex]\text{So, the growth rate increases over the interval}\ \bigg(-\infty, \dfrac{ln\ 4}{0.2}\bigg)[/tex]
