Question

Which one of the following is more accurate?
option a:20 X 10 to the power of -3 m
option b:200 X 10 to the power of -4 m
option c:2 X 10 to the power of -2m
option d:0.02m

Answer 1

option b: is correct

Answer 2

200  contains three significant digits.
20 contains two 
2 contains one 
0.02 contains one significant digit.

Obviously  200 * 10^ -4 m   is the most accurate specification of the length.