Final answer:
Encephalitozoon cuniculi is a microsporidian, a type of fungi-related parasite, answering the question with option (d) Parasite.
Step-by-step explanation:
Encephalitozoon cuniculi is a microorganism that has been recently reclassified as a microsporidian, which is a type of fungi-related parasite. Therefore, the correct answer to the question "Encephalitozoon cuniculi is a ______." would be (d) Parasite. Although it was once thought to be a protozoan, advances in genetic and molecular analyses have led to the reclassification of microsporidia as fungi or fungi-related organisms. This reclassification is due to the discovery that they share a common ancestor with fungi and possess some similar characteristics, despite their unique life cycles and parasitic nature.