Question

What is the longest subdivision in geologic time?

Answer 1

The PreCambrian era.

Explanation:

For decades geologists didn't know much about the period of time older than about 500 million years to the Earth's formation some 4.5 billion years ago - so, they simply called this time the "PreCambrian". More recently, more evidence has been found to subdivide the PreCambrian into small units. However, the Proterozoic is now still over 2 billion years of time and so, would be the longest.

This lack of evidence for older rocks is because few really old rocks are preserved in the rock record, as plate tectonics and the rock cycle, tend to plow rocks back into the mantel and we lose the evidence.