What is the measurement 111.009 mm rounded off to four significant digits?

a. 111mm

b. 111.0 mm

c. 111.01mm

d. 110mm

To 4 significant figures , the correct answer should be B. 111.0 mm.

111.009 mm
It has :
6 significant figures.
As the zero to the left of 9 is less than 5.
So we have to get rid of it , and it is cancelled and 9 with it also get cancelled.
So, it will become,
111.0 mm
It has 4 significant figures:
*3 nonzero digits
*1 trailing zero
So according to above explanation,
b. 111.0 mm, is the correct answer.