Which of the following is an example of a microorganism?

Viruses are a prime example of microorganisms because they are microscopic entities that are capable of infecting host cells and reproducing within them. Despite being composed of genetic material (either DNA or RNA) encased in a protein coat, viruses are unique in that they require a living host for replication, distinguishing them from cells, which can replicate independently.

Microorganisms are typically defined as tiny organisms that can only be seen under a microscope, and they include a wide range of entities such as bacteria, viruses, fungi, and some protozoa. The size of viruses generally ranges from about 20 to 300 nanometers, making them smaller than other microorganisms like bacteria.

The other options, while related to the human body and immune responses, do not fit the classification of microorganisms. White blood cells are a part of the immune system and are much larger than viruses or bacteria, functioning as essential defenders against pathogens. Platelets are cell fragments involved in blood clotting and are also significantly larger than viruses. Hormones are chemical messengers produced by glands in the endocrine system and do not have a cellular structure or the characteristics typical of microorganisms. In essence, viruses stand out in this context as they fulfill the criteria for being microorganisms.