Amoebiasis is caused by

The correct option is Entamoeba histolytica.

Step-by-Step Explanation:

1. Understanding Amoebiasis:
Amoebiasis (also known as amoebic dysentery) is a gastrointestinal infection characterized by symptoms such as abdominal pain, diarrhea, and bloody stools. It is primarily transmitted through the ingestion of food or water contaminated with fecal matter containing the cysts of the causative organism.

2. Identifying the Causative Agent:
The disease is caused by a protozoan parasite called Entamoeba histolytica. Once ingested, the cysts of this parasite pass into the intestine, where they release active trophozoites that invade the mucosal lining of the colon, leading to the characteristic symptoms of the disease.

3. Analyzing the Other Options:
- Toxoplasma gondii: This is a protozoan parasite that causes toxoplasmosis, an infection often associated with cats and contaminated undercooked meat, not amoebiasis.
- Ascaris lumbricoides: This is a giant intestinal roundworm that causes ascariasis, a helminthic infection, rather than amoebiasis.

Therefore, Entamoeba histolytica is the specific organism responsible for causing amoebiasis.