Parasite that is also a vector host is

The correct option is Bug.

To understand why this is the correct answer, let us break down the biological definitions of a parasite and a vector host:
1. Parasite: An organism that lives on or inside another organism (the host) and benefits by deriving nutrients at the host's host expense.
2. Vector Host: An organism (typically an arthropod) that transmits an infectious pathogen or parasite from one host to another.

Certain species of bugs, such as the kissing bug (triatomine bug), are ectoparasites because they feed on the blood of vertebrates. Simultaneously, they serve as vectors. For example, the kissing bug is a vector host for the protozoan parasite Trypanosoma cruzi, which causes Chagas disease in humans. When the bug feeds on host blood, it defecates near the bite, and the parasite enters the host's body through the bite wound or mucous membranes.

Let us evaluate the other options to see why they do not meet both criteria:
• Ascaris: It is an endoparasitic roundworm that lives in the human intestine. While it is a parasite, it does not act as a vector host.
• Fasciola: Commonly known as the liver fluke, it is a parasitic flatworm (endoparasite) but does not transmit other pathogens as a vector.
• House fly: It can mechanically transmit pathogens from filth to food, but it is not classified as a parasite because it does not live on or in a host to feed on its tissues or blood.