**Core Concept**
The Cavett test is a forensic analytical technique used to detect the presence of a specific substance in biological samples. In this context, it is utilized to identify the presence of alcohol in a person's system, which is crucial in forensic investigations and legal proceedings.

**Why the Correct Answer is Right**
The Cavett test is specifically designed to measure the concentration of ethanol in blood or other biological samples. This is achieved through a chemical reaction that involves the oxidation of ethanol to acetaldehyde, which is then measured using a spectrophotometer. The Cavett test is based on the principle of enzymatic oxidation, where the enzyme alcohol dehydrogenase (ADH) catalyzes the conversion of ethanol to acetaldehyde. The rate of this reaction is directly proportional to the concentration of ethanol present in the sample. This makes the Cavett test a reliable and accurate method for determining blood alcohol levels.

**Why Each Wrong Option is Incorrect**
**Option A:** The Cavett test is not used for detecting opiates, as it is specifically designed for ethanol detection. Opiate detection typically involves other analytical techniques such as immunoassays or chromatography.
**Option C:** Barbiturates are not detected using the Cavett test, which is focused on ethanol. Barbiturate detection often involves other methods such as thin-layer chromatography (TLC) or gas chromatography-mass spectrometry (GC-MS).
**Option D:** Cocaine detection is not related to the Cavett test, which is used for ethanol detection. Cocaine detection typically involves other analytical techniques such as immunoassays or chromatography.

**Clinical Pearl / High-Yield Fact**
In forensic medicine, it is essential to note that the Cavett test can only detect the presence of ethanol, not its effects or the level of intoxication. Therefore, a positive result does not necessarily imply impairment or driving under the influence (DUI).

**✓ Correct Answer: B. Alcohol**