Answer
A. The calculated molar mass will be too high.
Step-by-step explanation
The Dumas method is a method appropriate to determine the molecular weights of volatile organic substances that are liquids at room temperature.
So when using this method to determine the molar mass of a volatile material, and a student fails to wipe off the outside of the test tube, it implies the calculated molar mass will be too high. This is because some particle will have stick to