Which unit of geologic time is the oldest? Cenozoic Mesozoic Ordovician Precambrian

The oldest unit of geologic time is Precambrian.

Explanation:

The period of Precambrian prolonged from 540 to 490 millions of years ago. Likewise for Ordovician, 490 to 443 million year ago, whereas for Mesozoic period extends from 248 to 65 million year ago. At last, Cenozoic time lies between 65 million to 10,000 years ago.

The organisms in each period are as follows,

Precambrian time - Sponges, Trilobites. Ordovician - Jawless fishes, Gastropods, etc. Mesozoic period - Early mammals, Bony fishes. Cenozoic period - Human beings, Mammoths, etc.

D. Precambrian