If the reaction doesn't go to completion, you will have aldehyde starting material present. how would you identify this impurity most easily by 1h nmr?

 - If the reaction does not go to completion, the presence of aldehyde can be identify easily by looking at the aldehydic proton peak in the range of 10- 9 ppm (Chemical shift value).
- Aldehydic proton has a high value of chemical shift due to magnetic anisotropy so when looking at the most de-shielded proton we will found a signal or peak at 9-10 ppm corresponding to the proton of aldehyde 

Most likely a is the answer - because  you generate   38 atp's but also consume an atp.   each atp gives approx. less than 4 kcal so probably a is the closest answer.