Final answer:
During the 1960s, the illegal drug most likely to be used was marijuana, which became a hallmark of the counterculture movement and was widely used despite its illegal status.
Step-by-step explanation:
The illicit drug of choice during the 1960s was marijuana (cannabis). This period was characterized by a cultural shift towards more liberal social values and the widespread use of various psychoactive substances. Despite being illegal, marijuana was commonly used recreationally and for its psychoactive effects which alter consciousness and can induce a 'high'. Of the options provided, marijuana saw significant use during this era, not just for recreation but also for its association with the counterculture movement. LSD also rose to prominence due to its potent hallucinogenic properties and the consequential changes in perception and mood it produces, but marijuana still held precedence in terms of widespread illicit use.