Final answer:
The Hadean era spanned from 4.6 to 3.8 billion years ago, starting with Earth's formation and characterized by harsh conditions. It is significant as the period during which Earth began cooling and the earliest life forms emerged(option a).
Step-by-step explanation:
The Hadean era is a period in Earth’s geologic history that spans from the formation of the planet to the start of the Archean eon. Specifically, the Hadean era started about 4.6 billion years ago, when Earth formed, and lasted until about 3.8 billion years ago. This corresponds to option 'a', which states the Hadean era spanned from 4.6 to 3.8 billion years ago (Ga).
During this time, Earth was undergoing its initial stages of cooling and the environment was extremely harsh. Despite the challenging conditions, the foundations for life were being laid. It is this period that has marked the emergence of simple cells and, eventually, the earliest forms of life.