Answer :

To determine the number of significant figures in the measurement 16.400 g, we should follow the rules for counting significant figures:

1. All non-zero digits are significant.
2. Any zeros between significant digits are also significant.
3. Leading zeros (zeros to the left of the first non-zero digit) are not significant.
4. Trailing zeros (zeros to the right of both a non-zero digit and a decimal point) are significant.

Now let's apply these rules step-by-step to the measurement 16.400:

- 1: The digit 1 is a non-zero digit and therefore, it is significant.
- 6: The digit 6 is a non-zero digit and therefore, it is significant.
- 4: The digit 4 is a non-zero digit and therefore, it is significant.
- The two zeros following the 4: These zeros are trailing zeros to the right of a non-zero digit and also follow a decimal point, so they are significant.

Thus, in the number 16.400, we have:
- The digit 1 is significant.
- The digit 6 is significant.
- The digit 4 is significant.
- The first 0 is significant.
- The second 0 is significant.

Adding these up, we have a total of 5 significant figures.

Therefore, 16.400 has [tex]\(\boxed{5}\)[/tex] significant figures.