Final answer:
Alcohol is synergistic with Seconal, a barbiturate, because both substances act as central nervous system depressants and can dangerously enhance each other's effects.
Step-by-step explanation:
Among the options provided, alcohol is synergistic with Seconal, a barbiturate. Seconal can enhance the central nervous system (CNS) depressant effects of alcohol, and when taken together, they can significantly increase the risk of serious side effects including respiratory depression, coma, or even death. Drugs like heroin, which is an opioid, also have CNS depressant effects and can be synergistic with barbiturates. Unlike alcohol or opioids, marijuana, cocaine, and LSD work on different neurotransmitter systems and are not typically considered to be synergistic with barbiturates in terms of their sedative effects.