Answer is D.
As per the question, it is quite clear that the danger of a microbe to human being will be directly proportional to potency and growth. At the same time it is inversely proportional to toxicity, defined as( more dangerous will a microbe be if lesser of its milligram is required).
So, $\text{Level Of Danger (D)} \propto \text{Growth (G)} \propto \text{Potency (P) }\dfrac{1}{\propto} \text{ Toxicity (T)}$
$D = \dfrac{KGP}{T}$
where K is contant of proportionality.
So level of danger will be maximum for $S.$
Given by,
$DS =\dfrac{0.8\times \large\pi(10)^{2}}{200}$
$\qquad= 1.256$
Similar calculations for $D{_P} , D{_Q} , D{_R}$ can be done. Which will consequently lead to $D{_S}$ being the most dangerous and hence will be targeted first.