Without googling, I guess: 2. Hydrogen

It's the lightest element and very often elements are fused into higher elements in cosmic events (or in stars), so I assume it's hydrogen which has to be abundant in order for that to keep going?

Oxygen is obviously not abundant, as life was here before oxygen even started joining the other elements on Earth.

Calcium has no logic to it at all, so nah....

And helium would be the second most logical choice, but still it would be kinda strange.