1. Write the given masses

   $m_1 = 435.42\,\text{g}, \quad m_2 = 226.3\,\text{g}, \quad m_3 = 0.125\,\text{g}$

2. Add them normally

   $m_1 + m_2 + m_3 = 435.42 + 226.3 + 0.125$

   First, $435.42 + 226.3 = 661.72$

   Then, $661.72 + 0.125 = 661.845\,\text{g}$

3. Apply the rule for addition with significant figures

   For addition/subtraction, the result is rounded according to the least number of decimal places among the given measurements.

   • $435.42$ has 2 decimal places
   • $226.3$ has 1 decimal place
   • $0.125$ has 3 decimal places

   So the final answer must be reported to 1 decimal place.

4. Round the sum

   Exact sum: $661.845\,\text{g}$

   Rounded to 1 decimal place: $661.8\,\text{g}$

5. Check options

   • A: $661.845\,\text{g}$ → not rounded properly
   • B: $661.84\,\text{g}$ → 2 decimal places, incorrect
   • C: $662\,\text{g}$ → rounded to nearest integer, too coarse
   • D: $661.8\,\text{g}$ → correct

Therefore, the correct option is D.