Which of the following geologic time segments is the oldest?

Precambrian era is the oldest geologic time segments.

Explanation:

The Precambrian era is the most ancient part of Earth's history. It accounts for about major part of the Earth's geological time. It constitutes the time period from the origin of the Earth up until the Cambrian explosion. Amongst the options that are given, Cenozoic era comes last and before that comes Mesozoic. The Ordovician era also comes later than the Precambrian era. The earliest forms of life have found to be existent during the Precambrian era.