Answer:
A millimeter is 
 meter.
 meter.
Explanation:
We have been given an incomplete sentence. We are asked to fill in the blank for given statement.
A millimeter is ______ meter(s).
We know that 1 meter equals 1000 millimeter.
This means that 1 millimeter is 
 part of a meter, therefore, option D is the correct choice.
 part of a meter, therefore, option D is the correct choice.