Answer
$(D)$
Work Step by Step
Goal: $\displaystyle \quad f(x)=\frac{N}{1+Ab^{-x}}$,
where $N$ = limiting value = over $30$ (B and C are eliminated).
Cases A and D both have A=$0.06$, so we don't observe $f(0)$.
The graph is decreasing, so $b\lt 1,$ and cases A and D both have this.
We have a choice of $b$ being either $0.02$ or $0.7$.
Since f(x) is roughly exponential for small x, and we have for $x\approx 0$, the value $f(0)\approx 32$.
Increasing x by 1 (from 0 to 1), we expect $f(x)$ to decrease by factor $b.$ The graph suggests a factor $0.7$ rather than $0.02$, ($0.02$ would cause a drop to almost zero for x=1), so our choice is $(D)$.
