What drug is referred to by someone saying they are going to do a line?

When someone refers to doing a line, they are typically talking about cocaine. Cocaine is often processed into a white powder that can be snorted through the nose, and users commonly form it into lines for this purpose. The terminology associated with cocaine use, such as "doing a line," is widely recognized in popular culture and among users, making it a distinct phrase linked specifically to cocaine consumption.

In contrast, while heroin is an opioid that can be injected or smoked, it is not commonly associated with the term "line." Methamphetamine (meth) can also be ingested in various ways, including snorting, but is not typically referenced in the same manner as cocaine. Thus, the phrase "doing a line" is most directly connected to the use of cocaine, making it the correct answer.