Answer :
The correct answer is due to incorrect position of thermometer .
Distillation is process of separating two volatile liquids on the basis of their boiling points .In involves the following set up ( shown in image )
Since the Boiling point is found is 116-117 C which is near to 124 C , also the purity was checked and thermometer was calibrated .
So the only error could be possible is position of thermometer .
The position of bulb of thermometer is very important to get an accurate reading of boiling point .The vapor so formed should touch the side walls of bulb as soon as they are formed .
May be the bulb of thermometer was kept far from the vapors formed by liquid so, some of the vapor before reaching to bulb start condensing and their Temperature decreases .
Hence , the temperature shown by thermometer was decrease than actual boiling point due to incorrect position of thermometer.
