Question

when the earth was formed about 4.6 billion years ago, the atmosphere was void of what gas?
hydrogen
carbon dioxide
water vapor
oxygen

Explanation:

When Earth formed ~4.6 billion years ago, its early atmosphere lacked free oxygen ($O_2$). Early Earth's atmosphere had gases like hydrogen, carbon dioxide, and water vapor from volcanic outgassing, but oxygen (in molecular form) was absent until photosynthetic organisms evolved later.

Answer:

D. oxygen