Question

cnidaria - shape of life
approximately how long ago did cnidarians first appear?
100 million years ago
50,000 years ago
500 million years ago
2 billion years ago

Explanation:

Cnidarians are an ancient group of animals. Fossil evidence and scientific research indicate that they first appeared around 500 million years ago. 100 million years ago is too recent for their first appearance. 50,000 years ago is extremely recent (human - like species have been around for a few million years at most). 2 billion years ago is too early as complex multicellular animals like cnidarians did not exist then.

Answer:

500 million years ago