Which of the following describes usually single-cell microscopic organisms such as bacteria, molds, and fungi?

The term that describes usually single-cell microscopic organisms such as bacteria, molds, and fungi is microorganisms. Microorganisms are a diverse group of organisms that can only be seen under a microscope and include not just bacteria, molds, and fungi, but also viruses and protozoa. They play crucial roles in various processes, including decomposition, fermentation, and as part of the human microbiome.

Other options, while relevant in biological or nutritional contexts, refer to different concepts. Macromolecules are large, complex molecules such as proteins, fats, and carbohydrates, which are essential components of biological systems but do not refer to living organisms. Antigens are substances that can induce an immune response in the body, often associated with pathogens, but they are not organisms themselves. Pathogens specifically refer to microorganisms that cause disease, which is a subset of the broader category of microorganisms. Therefore, the correct answer clearly identifies the category that encompasses bacteria, molds, and fungi: microorganisms.