The correct answer is option 2: Mycoplasma. Mycoplasma is a genus of bacteria that are characterized by having the smallest known living cell size. These bacteria lack a cell wall and are therefore able to adopt different shapes and sizes. Due to the absence of a cell wall, Mycoplasma cells are very flexible and can squeeze through small spaces, which allows them to invade and colonize various host tissues.

Option 1: Bacterium is a broad term that encompasses many different types of bacteria, some of which have larger cell sizes compared to Mycoplasma.

Option 3: Virus is not a living cell. Viruses are considered to be acellular entities that require a host to reproduce. They are much smaller than cells and consist of genetic material (DNA or RNA) surrounded by a protein coat.

Option 4: Yeast is a type of fungus and is composed of larger eukaryotic cells. Yeast cells are typically larger in size compared to Mycoplasma cells.

In summary, Mycoplasma bears the smallest living cell among the options provided.