A) True. Geologists do subdivide eras into periods. The geologic timescale is divided into hierarchical units, starting with the largest unit, called an eon. Each eon is further divided into eras, and each era is further divided into periods. This subdivision allows for a more detailed and organized understanding of Earth's history.
16- C) Eon. Among the options provided, the longest division on the geologic timescale is the eon. Eons represent the largest units of time on the geologic timescale and are further subdivided into eras, which are then subdivided into periods. Eons encompass billions of years of Earth's history, making them the longest divisions.