Final answer:
Morphine is cross-tolerant with heroin because they are both opioids and affect similar neurotransmitter systems in the body, leading to cross-tolerance.
Step-by-step explanation:
The substance that is cross-tolerant with heroin is c) morphine. Heroin, also known as diacetylmorphine, is metabolized into morphine in the body. Therefore, both substances are opioids and have similar effects on the brain and body, leading to cross-tolerance. Cross-tolerance occurs when tolerance to the effects of one drug causes tolerance to another drug within the same category. Other options listed, such as alcohol, marijuana, psilocybin, and LSD, are not opioids and they affect different neurotransmitter systems; hence they are not cross-tolerant with heroin.