Answer:
The most suitable answer to the given question out of the given options is:
The earth's gravity was weak
Explanation:
One of the reason to explain that much of the early helium and hydrogen escaped into space from the earth's atmosphere is because the escape velocity of helium and hydrogen is much higher than the critical escape velocity of earth.
Another reason is that Helium and Hydrogen are much lighter in weight and can't be pulled into the atmosphere by weak gravity of earth.