**Core Concept**
Speed ball is a term used to describe a mixture of two substances that are often abused together, leading to unpredictable and potentially deadly consequences. In this case, the combination involves cocaine, a powerful stimulant, and another substance that enhances its effects. The underlying principle being tested is the pharmacological interaction between these two substances.

**Why the Correct Answer is Right**
The correct answer is heroin, an opioid, which is often mixed with cocaine to create a speed ball. When cocaine and heroin are combined, the stimulant properties of cocaine can mask the initial effects of heroin, leading users to take higher doses, increasing the risk of overdose and death. Heroin works by binding to opioid receptors in the brain, releasing endorphins, and producing feelings of euphoria. This interaction highlights the dangers of polypharmacology and the unpredictable effects of mixing different substances.

**Why Each Wrong Option is Incorrect**
**Option A:** Methamphetamine is another stimulant that might be mixed with other substances, but it is not typically combined with cocaine to create a speed ball.
**Option B:** Opioid antagonists like naloxone or naltrexone are used to reverse opioid overdose, but they are not mixed with cocaine to create a speed ball.
**Option C:** Benzodiazepines like diazepam or alprazolam are sedatives that might be abused separately, but they are not typically combined with cocaine to create a speed ball.

**Clinical Pearl / High-Yield Fact**
Remember the acronym "HARM" to associate with the dangers of speed balling: Heroin, Amphetamines, and other substances can lead to unpredictable and potentially deadly consequences.

**Correct Answer: D. Heroin.**