B

To determine the number of significant digits in 200.40, we follow these rules:
1. **All non-zero digits are significant**. Hence, the digits '2' and '4' are significant.
2. **Any zeros between significant digits are also significant**. In this case, the two zeros in '200' are between significant digits and therefore significant.
3. **Leading zeros are not significant**. However, there are no leading zeros in '200.40'.
4. **Trailing zeros in a decimal number are significant**. The zero after '4' in '200.40' is a trailing zero and is considered significant.
Thus, we count the significant digits: '2', '0', '0', '4', and the trailing '0', giving us a total of 5 significant digits.
Therefore, the answer is B: 5.