**Core Concept**
Speedball is a colloquial term for a mixture of two substances, one of which is a potent stimulant, and the other a potent opioid, often used recreationally to produce a synergistic effect. This combination can lead to unpredictable and potentially fatal outcomes.

**Why the Correct Answer is Right**
The correct answer is heroin, an opioid, mixed with cocaine, a powerful central nervous system stimulant. When combined, these substances can produce a rapid increase in dopamine levels in the brain, leading to intense euphoria and increased risk of respiratory depression, cardiac arrest, and overdose. The opioid component, heroin, binds to mu-opioid receptors in the brain, while cocaine blocks the reuptake of dopamine, serotonin, and norepinephrine, resulting in an exaggerated stimulatory effect.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the opioid component of the speedball mixture. While other opioids like fentanyl or morphine could be mixed with cocaine, the term "speedball" specifically refers to heroin.
**Option B:** This option is incorrect because it does not accurately describe the speedball mixture. Cocaine is a stimulant, but mixing it with another stimulant like amphetamine would not produce the same synergistic effect as combining it with an opioid.
**Option C:** This option is incorrect because it does not specify the opioid component of the speedball mixture. While other opioids like oxycodone or hydrocodone could be mixed with cocaine, the term "speedball" specifically refers to heroin.
**Option D:** This option is incorrect because it does not accurately describe the speedball mixture. Cocaine is a stimulant, but mixing it with another depressant like benzodiazepine would not produce the same synergistic effect as combining it with an opioid.

**Clinical Pearl / High-Yield Fact**
Speedball users are at increased risk of developing acute respiratory depression, cardiac arrhythmias, and seizures due to the unpredictable interaction between the stimulant and opioid components. It is essential for healthcare providers to be aware of this high-risk behavior and provide timely medical intervention in case of an overdose.

**Correct Answer: C. Heroin**