Over 90% of all atoms in the universe are a) hydrogen b) oxygen c) carbon d) nitrogen

The right answer is Hydrogen.

Hydrogen is the most abundant element in the Universe: 75% by mass and 92% by number of atoms. On Earth, if it represents only 0.22% of the atoms, it constitutes 63% of the atoms of the human body.

It is present in large quantities in stars and gaseous planets; it is also the main component of nebulae and interstellar gas.

It is the first element of the periodic table (it bears the number 1, indicated at the bottom left of the symbol H), hydrogen is the simplest chemical element since it consists only of a proton and an electron.