**Core Concept**
The phenomenon described in the question is a classic example of synesthesia, a neurological condition where one sense is simultaneously perceived as if by one or more additional senses. In this case, the boy is experiencing synesthesia due to the effects of a specific psychoactive substance.

**Why the Correct Answer is Right**
The substance most likely responsible for this condition is lysergic acid diethylamide (LSD), a potent hallucinogenic drug known to induce synesthesia in many users. LSD acts by binding to serotonin receptors in the brain, particularly the 5-HT2A receptor subtype, which is involved in the regulation of sensory processing and perception. This binding causes an alteration in the normal processing of sensory information, leading to the characteristic cross-over of sensory perceptions.

**Why Each Wrong Option is Incorrect**
* **Option A:** Cannabis, a commonly abused substance, is known to produce a variety of psychoactive effects, but synesthesia is not typically associated with its use.
* **Option B:** Cocaine, a powerful stimulant, is more likely to produce effects related to increased energy and alertness, rather than synesthesia.
* **Option C:** Amphetamines, a class of central nervous system stimulants, can produce a range of psychological effects, but synesthesia is not a characteristic symptom.

**Clinical Pearl / High-Yield Fact**
LSD-induced synesthesia can be a challenging condition to diagnose, as the symptoms can be similar to those of other neurological or psychiatric disorders. It's essential for clinicians to consider the patient's substance use history when evaluating such cases.

**Correct Answer:** C. LSD