**Core Concept**
The question requires knowledge of definitive hosts for various parasites. A definitive host is the host in which the parasite reaches maturity and reproduces sexually, leading to the production of infective stages. This concept is crucial in understanding the life cycle of parasites and the development of effective treatment strategies.

**Why the Correct Answer is Right**
The correct answer is the parasite that completes its life cycle in humans, where it reaches maturity and reproduces sexually. The definitive host for tapeworms is humans, where they attach to the intestinal wall, absorb nutrients, and reproduce, producing eggs that are excreted in the feces. This process occurs in the human intestine, making humans the definitive host for tapeworms.

**Why Each Wrong Option is Incorrect**
* **Option A:** Malaria is caused by Plasmodium parasites, which have a complex life cycle involving both a human host (definitive) and a mosquito vector (intermediate). Humans are not the definitive host for malaria parasites.
* **Option B:** Filariasis is caused by filarial parasites, which also have a complex life cycle involving both a human host (definitive) and a mosquito vector (intermediate). Humans are not the definitive host for filarial parasites.
* **Option C:** Measles is a viral infection caused by the measles virus, not a parasite. Therefore, it does not have a definitive host in the context of parasitology.

**Clinical Pearl / High-Yield Fact**
When considering the life cycle of parasites, it's essential to distinguish between definitive and intermediate hosts. This knowledge is crucial for developing effective treatment strategies and preventing the spread of parasitic diseases.

**Correct Answer: D. Tapeworm.