Which is the most likely the unit to measure the size of a typical mature yeast cell?

The answer is micrometer.

Micrometer (μm) or micron is unit of length which is 10⁶ times smaller than meter. Yeasts are unicellular organisms that belong to the kingdom of Fungi. Yeast cells vary in sizes, depending on species or environment conditions, but all cells can be measured in micrometers. Usually, yeast cells are 3-4 μm, but some species can reach size of 40 μm.