Answer:
Centimeters.
Step-by-step explanation:
There are only 2 viable answers here. This is due to the fact that moles are used not to measure diameters but instead the number of atoms.
Similarly, milligrams are not used to measure diameters but instead small volumes.
This leaves us with kilometers and centimeters, and since all 3 objects are more easily representable in centimeters than kilometers, the answer must be centimeters. One example would be the diameter of a ping pong ball, which is approximately 4cm, or in other words, 0.00004km, with 4cm being much more easy to represent as it overall involves less digits.