Answer:
 -0.89%
Explanation:
Assuming a proportional relationship between EMF and Celsius temperature, the measured value of EMF at 100 °C will be interpreted as a temperature of ...
 T/5.268 = 400/21.846
 T = 5.268(400/21.846) ≈ 96.457 . . . . °C
The error as a fraction of full scale is ...
 (96.457 -100)/400 × 100% ≈ -0.886%
The error at 100 °C is about -0.89% of full scale.