The unit of length that a microscope best helps to see is 1 millimeter. Option B is correct.
Microscopes are instruments used for viewing objects that cannot be seen with naked eyes.
The unit length of an object that a microscope can view should be a length that is very small.
For the first option, 1 kilometer = 1000m, this measurement is quite large hence cannot be viewed under a microscope. The same goes with 1 meter and 1 centimeter.
The unit of length that a microscope best help to see is 1 millimeter
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