The phrase "gateway drug" typically refers to a substance that may lead to the use of more dangerous or addictive drugs. In this context, Corinne’s parents are likely concerned that her use of a less dangerous drug could increase the likelihood of her trying more harmful substances in the future.
Therefore, the most likely interpretation among the options provided is:
Smoking cigarettes will lead to her trying more dangerous drugs.
This reflects the common understanding that cigarette smoking is often viewed as a gateway to using other, more potent drugs.