**Core Concept**
The question is testing the student's knowledge of postmortem changes and the characteristic findings in cases of poisoning with certain substances. The dark brown postmortem staining and garlic odor in the stomach are specific signs that can help identify the poison involved.

**Why the Correct Answer is Right**
The characteristic dark brown postmortem staining and garlic odor in the stomach are indicative of arsenic poisoning. Arsenic is a potent toxin that can cause a range of symptoms, including nausea, vomiting, and abdominal pain. When ingested, arsenic is absorbed into the bloodstream and distributed throughout the body, where it can cause damage to various organs, including the liver, kidneys, and central nervous system. The garlic-like odor is due to the production of trimethylarsine gas, which is a metabolic byproduct of arsenic.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because the described symptoms do not match those of other common poisons such as cyanide or organophosphates, which typically cause a rapid onset of symptoms and a distinct odor.

**Option B:** This option is incorrect because the described symptoms do not match those of carbon monoxide poisoning, which typically causes a cherry-red discoloration of the skin and mucous membranes, rather than a dark brown postmortem staining.

**Option C:** This option is incorrect because the described symptoms do not match those of opioids, which typically cause respiratory depression, miosis, and a lack of pupillary response, rather than a garlic-like odor.

**Clinical Pearl / High-Yield Fact**
In cases of suspected arsenic poisoning, it's essential to collect and preserve stomach contents, as well as tissue samples from the liver, kidneys, and other organs, for further analysis. This can help confirm the diagnosis and guide treatment.

**Correct Answer:** A. Arsenic poisoning.