The relevant formula is

$\begin{align*}I = A \cdot v_d \cdot q \cdot n\end{align*}$
where $A$ is the cross-sectional area of the wire, $v_d$ is the drift velocity of the charge carriers, $q$ is the charge of the charge carriers, and $n$ is the number density of the charger carriers. Thus,
$\begin{align*}v_d = \frac{I}{A \cdot q \cdot n} = \frac{100 \mbox{ A}}{\pi \cdot \left(0.01 \mbox{ m/s}\right)^2 \cdot \left(...$
Hence, answer (D) is correct.

Solution to 1986 Problem 9