The geological era about which we have the least information is called

Hadean

Explanation:

This a geologic eon period on earth dating to archean it begins with the formation of the planet about some 4.6 billion years ago. As data of 2016 ICS International commission on Stratigraphy describes its status as "informal". As geologist Preston cloud named this eon in 1972, before the earliest known rock formations on earth. Also called as the Priscoan period" or ancient rock period. Word Hadean also signified the Greek god of the underworld. The geologists believe that a sizeable quantity of water could have been the prime source of earth and that water molecules would have escaped due absence of earth gravity. It also asserted that the parts of the earth might have been disrupted due to the formation of the moon.

B. a graduated substance