The subdivision of the geologic time scale that represents the longest time span is called a

The subdivision of the 'geologic time scale' which represents the 'longest time span' is called an eon.

Explanation:

On the timescale of geologic history, an 'eon' is a period made up of 1 billion years. The one we are living in is the 5th eon. An 'era' which is a period shorter than eon lasts for hundreds of million years. Eras are divided in different 'geological periods'. An epoch is a geologic timespan of more than 3 million years.

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