According to **Heisenberg's Uncertainty Principle**:
\[
\Delta x \cdot \Delta p \geq \frac{h}{4\pi}
\]
where \( \Delta x \) is the uncertainty in position and \( \Delta p \) is the uncertainty in momentum.
The uncertainty in momentum is given by:
\[
\Delta p = m \cdot \Delta v
\]
Given that the velocity is **\( v = 50 \) m/s** with an **uncertainty of 2\%**, we calculate:
\[
\Delta v = \frac{2}{100} \times 50 = 1 \text{ m/s}
\]
Thus,
\[
\Delta p = m \times 1
\]
From Heisenberg's principle:
\[
\Delta x \geq \frac{h}{4\pi \Delta p}
\]
Substituting \( \Delta p = m \):
\[
\Delta x \geq \frac{h}{4\pi m}
\]
Since the uncertainty in speed is in **meters per second (m/s),** we express the uncertainty in position in **millimeters (mm)**:
\[
\Delta x \geq \frac{h}{4\pi m} \times 10^3 \ \text{mm}
\]
Hence, the correct answer is:
\[
\boxed{\frac{h}{4\pi m} \times 10^3}
\]