A gateway drug is one which precedes the use of illegal or illicit drugs. So, gateway drugs are usually the types of drugs people use before moving on to something heavier and far more dangerous. Many consider marijuana to be a gateway drug, because it can make you feel dependent on it and crave something more. The other options don't make much sense here, as prescription drugs are something your doctor gives you, tobacco-based drugs aren't that dangerous, and harmless drugs are, well, harmless.
