Answer:
b. The number of electrons
Explanation:
A "neutral atom" has a neutral charge. This means that its charge is equal to zero. In order for the charges to cancel out each other, the atom's positive charge should be equal to the negative charge. These being said, the number of electrons (negatively-charged) is then equal to the number of protons (positively-charged). Those atoms which are not neutral are called "ions." This means that they either have more or less electrons than the protons.
