What are single-cell organisms that grow in irregular masses, including molds, mildews, and yeasts, called?

The term used for single-cell organisms that grow in irregular masses, including molds, mildews, and yeasts, is "fungi." Fungi are a distinct group of organisms that can be unicellular or multicellular and are known for their ability to decompose organic matter. This characteristic makes them incredibly important to ecosystems.

Molds and mildews are examples of multicellular fungi, while yeasts are primarily unicellular. They reproduce by forming spores and thrive in damp or decaying organic material. Fungi play a crucial role in the nutrient cycle, breaking down complex organic substances into simpler ones, allowing for nutrient absorption by other organisms.

The other choices refer to different types of microorganisms. Bacteria are unicellular organisms that lack a nucleus and have a different cellular structure compared to fungi. Viruses are not classified as living organisms; they require a host cell to replicate and do not possess the cellular machinery of fungi or bacteria. Parasites are organisms that live in or on a host, often causing harm, and can include some fungi, but generally refer to more complex life forms, which distinguishes them from the single-cell organisms specifically defined in the question.