How many significant figures are in 675000 m
6
4
3
5

There are 3 significant figures in 675000

Explanation:

The rules for significant figures:

- Non zero digit (from 1 upward) are always significant

- Trailing zero are not significant

- but zero in the presence of decimal are significant

In this case, the trailing zeros are not significant thus only the first 3 non zero digit are significant.  

increased stability in the completion of fluorine's outer shell.