Why is helium less reactive than hydrogen?
a.
each helium atom has four neutrons, but each hydrogen atom only has one neutron.
b.
each helium atom has a full protonic power level, but each hydrogen atom only has one proton.
c.
each helium atom has two electron energy levels, but each hydrogen atom only has one electron energy level.
d.
each helium atom has a full valence electron shell, but each hydrogen atom only has one valence electron.

D.

Each helium atom has a full valence electron shell, but each hydrogen atom only has one valence electron.

Explanation:

Atoms are made of protons, neutrons and electrons. Protons and neutrons are in the nucleus and electrons orbit in energy levels. Chemical properties of elements depend mostly on the number of electrons in the valence / outermost energy shell.

Elements that have an incomplete energy shell are more reactive as they try to gain a complete outer shell electron configuration either by losing electrons or gaining electrons from the valence energy shell.

He is a noble gas, which means it has a full valence electron shell therefore it is inert. H has one electron in the outer shell therefore its reactive by gaining or losing the outermost electron.

therefore H is more reactive than He as it has an incomplete valence electron shell.