Option (D) 3.8 billion is correct because the textbook clearly states that 'Sometime around 3,800 million years ago, life began to evolve.' Additionally, it mentions that 'It can be assumed that life began to evolve sometime 3,800 million years ago.' Since 3,800 million years equals 3.8 billion years, this is the correct answer. Option (A) 13.7 billion refers to the age of the universe, not life on Earth. Option (B) 3.8 million is incorrect as it is too recent. Option (C) 4.6 billion refers to the age of the Earth itself, not the appearance of life.