Answer
$1584.9$, or (I)
Work Step by Step
We're looking for the value of $x$ where $M(x) = 6.2$
$ 6.2 = \log(\frac{x}{0.001})$
Take $10$ to the power of both sides:
$1584893 = \frac{x}{0.001}$
Multiply both sides by $0.001$:
$x = 1584.89$