25) A

The top of the column is the location where the compound with the lowest boiling point separates. This needs to be 68ºC. 

The temperature in the flask should neither be 68ºC nor 98ºC. If it was 68ºC, then this would mean that hexane would just start boiling. However, this cannot be true as we have obtained some distillate. It cannot be 98ºC as this would mean that heptane would start boiling. This is not true. So it has to be 83ºC. 

Hence the answer is A.