Final answer:
Protozoa are the single-celled eukaryotic organisms found in the bloodstream and intestinal tract of horses. They share some traits with animals and exhibit a variety of movement mechanisms. Protozoa can be parasitic, like Giardia or Plasmodium species.
Step-by-step explanation:
Protozoa are single-celled animals that occur in the bloodstream and intestinal tract of horses. Therefore, the correct answer to the question is (b) Protozoa.
Protozoa are single-celled eukaryotes that share some traits with animals, such as the ability to move. They can move by flagella, cilia, or pseudopodia, or they may not move at all. Protozoan parasites like Giardia or Plasmodium species are common examples that adapt through evolution to a parasitic way of life in hosts, including humans.
It's important to note that while protozoa are single-celled, they are categorized as eukaryotic cells because they have a membrane-enclosed nucleus, unlike prokaryotic cells like bacteria and archaea.