Final answer:
By the time the universe was 1/3 of its current age, only a small fraction of hydrogen and helium had been converted into heavier elements, with the correct answer being D) 2%.
Step-by-step explanation:
The question asks how much of the hydrogen and helium in the universe had been converted into heavier elements when the universe was 1/3 of its current age, around the time Earth formed. The Big Bang produced primarily hydrogen and helium, with hydrogen making up about 75% of the universe's mass and helium about 25%. Over time, stars began to convert these lighter elements into heavier ones through nuclear fusion.
According to the provided information, only about one-half of 1% of the critical density of the universe is found in stars, which are the main factories for heavier elements. Given that most of the universe's ordinary matter still consists of hydrogen and helium, we can infer that at the point when Earth formed, which is significantly later than the first few minutes when only hydrogen, helium, and a small amount of lithium were formed, a relatively small fraction of hydrogen and helium would have been processed into heavier elements.
Considering the slow process of star formation and subsequent element creation, as well as the dominance of light elements still evident, it is reasonable to conclude that the answer is D) 2%, signifying that a small fraction of hydrogen and helium had been converted into heavier elements by the time the universe was 1/3 of its current age.