1. The problem states that the length of a sofa is measured as 100 cm to 2 significant figures. 2. This means the true length could be between 99 cm and 101 cm, since 100 cm rounded to 2 significant figures is any value from 99.5 to 100.5 cm. 3. The maximum absolute error is half the unit of the last significant figure, here the last digit is in the ones place, so the error is approximately 0.5 cm. 4. Percentage error is calculated as $$\text{Percentage error} = \left(\frac{\text{Absolute error}}{\text{Measured value}}\right) \times 100 = \left(\frac{0.5}{100}\right) \times 100 = 0.5\%$$ 5. Therefore, the percentage error of the measured length is 0.5%. The correct answer is B. 0.5%