Helium is the second most abundant element in the universe, but it is rare in Earth’s atmosphere. Most of the helium used to fill balloons and blimps must be extracted from Earth’s crust. How do you think this helium formed?

Explanation:

Helium is produced in radioactive process that occurs in deep and underground formations. They differ from other atmospheric gases. These radioactive decay of heavier nuclides that are unstable started about the time when the earth was formed.

i think it's letter a.