Answer:
B. 1.2×10^-3 mm
Step-by-step explanation:
What constitutes an "appropriate" unit depends on the use of the number. Since we're not told of any particular use, we might choose the smallest of the available units: mm.
1.2×10^-3 mm
_____
Additional comment
The most appropriate unit would be the one that eliminates the power-of-ten multiplier:
= 1.2 µm
