Based on fossil evidence, about how long ago did the first
single-celled life form appear on earth?
130 million years ago
1.5 billion years ago
2.5 billion years ago
4.1 billion years ago

The correct answer will be option-4.1 billion years ago.

Explanation:

Earth the only planet in the universe which harbors life was formed around 4.5 billion years ago.  

Organic molecules formed from the inorganic molecules started aggregating and formed the first single-celled organisms which were microscopic. The fossil evidence found in the deep rocks and hydrothermal vents in the ocean suggests that single-celled organisms were formed around 3.5-4.2 billion years ago.

Thus, option-4.1 billion years ago is the correct answer.