Select the most appropriate option to fill in blank no. 1.
Show answer & explanation
Correct answer: A
Explanation
The correct answer is option 1 i.e. primary.
Context: The sentence is discussing nicotine and identifying its role within tobacco smoke. The focus is on what makes tobacco addictive, and among all the substances present, nicotine is emphasized as the main one responsible for addiction.
Primary means main or chief, which fits well here.
Hence, the chosen option will be correct.