Final answer:
Two billion years ago, the Sun had more hydrogen and less helium compared to today because its core has been fusing hydrogen into helium over time. The Sun's composition changes as it ages due to the process of nuclear fusion.
Step-by-step explanation:
The chemical composition of the Sun has changed over time due to the process of nuclear fusion in its core. Around 2 billion years ago, the Sun had more hydrogen and less helium compared to its present composition. The Sun, like other stars, generates energy in its core by fusing hydrogen atoms into helium through nuclear fusion. Over time, this leads to a decrease in the proportion of hydrogen and an increase in the proportion of helium.
When the Sun took its place on the zero-age main sequence approximately 4.5 billion years ago, it was composed predominantly of hydrogen and some helium. As the Sun ages and continues fusing hydrogen into helium, we can expect that two billion years ago, the Sun had more hydrogen than it does now. Therefore, the correct answer to the question is that the Sun had more hydrogen and less helium 2 billion years ago compared to today (Option a).