**Core Concept**
The question pertains to the treatment of luminal infection caused by *Entamoeba histolytica*, a protozoan parasite that infects the gastrointestinal tract. The treatment should target the parasite in the lumen of the intestine.

**Why the Correct Answer is Right**
The treatment of choice for luminal infection of *E. histolytica* is metronidazole, which is a nitroimidazole antibiotic. Metronidazole works by entering the cells of the parasite, where it is activated by the enzyme thioredoxin reductase. This activation leads to the formation of free radicals that damage the DNA of the parasite, ultimately causing its death. Metronidazole is effective against both the trophozoite and cyst forms of the parasite.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct treatment for luminal infection of *E. histolytica*. Luminal infections require a different treatment approach compared to invasive amoebiasis.
**Option B:** This option is incorrect because it is a treatment for bacterial infections, not protozoan infections like *E. histolytica*.
**Option C:** This option is incorrect because it is a treatment for viral infections, not protozoan infections like *E. histolytica*.

**Clinical Pearl / High-Yield Fact**
It is essential to note that metronidazole is not effective against the cysts of *E. histolytica* that are found in the stool. Therefore, a luminal amoebicide such as iodoquinol or paromomycin is often used in combination with metronidazole to ensure complete eradication of the parasite.

**Correct Answer: D. Metronidazole**