**Core Concept**
The question is testing the concept of parasite life cycles, specifically the number of hosts required for completion. In parasitology, the life cycle of a parasite can be categorized based on the number of hosts it requires, which is crucial for understanding the transmission dynamics and control of the disease.

**Why the Correct Answer is Right**
For the completion of its life cycle, _Toxoplasma gondii_ requires two hosts: a definitive host (cats) and an intermediate host (mammals). Once ingested, the oocysts excreted by cats are contaminated with food or water, which is then ingested by intermediate hosts like humans. In the intermediate host, the parasite undergoes asexual reproduction, forming tissue cysts. When an infected intermediate host is consumed by a cat, the parasite completes its life cycle. This dual-host life cycle explains why _T. gondii_ is a zoonotic parasite.

**Why Each Wrong Option is Incorrect**
* **Option A:** _Taenia solium_ (pork tapeworm) requires only one host, the human, for its life cycle. The larval stage (cysticercosis) develops in humans, but the adult stage (taeniasis) develops only in the human intestine. The pork acts as an intermediate host, but the parasite does not require two hosts.
* **Option B:** _Entamoeba histolytica_ is an amoeba that requires only one host, the human, for its life cycle. It does not require an intermediate host to complete its life cycle.
* **Option C:** _Taenia saginata_ (beef tapeworm) also requires only one host, the human, for its life cycle. The larval stage (cysticercosis) develops in cattle, but the adult stage (taeniasis) develops only in the human intestine.
* **Option D:** _Giardia_ spp. require only one host, the human, for its life cycle. It does not require an intermediate host to complete its life cycle.

**Clinical Pearl / High-Yield Fact**
The dual-host life cycle of _Toxoplasma gondii_ is a crucial aspect of its epidemiology, as it allows the parasite to infect a wide range of hosts, including humans. This understanding is essential for public health measures, such as preventing foodborne transmission and controlling the spread of toxoplasmosis in high-risk populations.

**Correct Answer: C. T. saginatad) Giardiae) Toxoplasma**