Which geologic era lasted the longest? . a. cenozoic. b. mesozoic. c. paleozoic. d. precambrian. i think, b.

D. Precambrian

The Precambrian Era is the longest geological era beginning 4.5 billion years ago and lasting approximately 4 billion years. The Precambrian era is generally separated into two eons the first of which being the Archaean Eon. spanning from around 4.5 billion years ago to 2.5 billion years ago, during this time air temperatures lowered and the water vapour condensed forming the first primeval oceans. This was a period of great volcanic activity when immense lava flows solidified both on the surface and below were transformed into rocks which were then carried by the first rivers to oceans where they were deposited at the bottom of the oceans forming the first sediments. The second eon was the Proterozoic Eon spanning from 2.5 billion years ago to 500 million years ago. 

The answer is c : ) hope this answered your question!