19. What is the longest subdivision in geological time? eon period epoch century

The longest subdivision in geological time is an eon. Geological time is structured in a hierarchical manner, starting from the longest intervals down to the shortest.

1. Eon: This is the largest division of geological time, which can span billions of years. For example, the Phanerozoic Eon covers the last 541 million years and is the most recent one in the geological time scale.

2. Era: Within an eon, there are eras, which are also significant spans of geological time.

3. Period: Each era is divided into periods, which further break down the geological timeline.

4. Epoch: Within a period, there are epochs, which are shorter time intervals than periods but still considerable in length.

5. Century: This is a human-made division of time and is the shortest unit in the provided options.

In summary, the correct answer to the question regarding the longest subdivision in geological time is an eon.