Final answer:
The cigarette additive used to enhance nicotine absorption is Ammonia. Therefore, the correct option is c.
Step-by-step explanation:
Nicotine is the active chemical constituent in tobacco, and it is used widely throughout the world for its stimulating and relaxing effects. The additive used to enhance nicotine absorption is Ammonia. Ammonia helps to increase the bioavailability of nicotine, allowing it to be absorbed more rapidly into the bloodstream.