At STP, a 1-liter sample of Ne (g) and a 1-liter

sample of Kr (g) have the same

(1) mass

(2) density

(3) number of atoms

(4) number of electrons

The answer you are looking for is (3) number of atoms.

Avogadro's law dictates this to be the case. This law states that when you have equal volumes of gases (1 L) at the same temperature and pressures (in this case STP), each vessel will have the same number of molecules.

The law states the same number of molecules, however, neon and krypton are noble gases which are found as single atoms rather than an entire molecule. So the law still applies to these gases.