Which quantity represents the number of protons in an atom?

(a) atomic number

(b) oxidation number

(c) number of neutrons

(d) number of valence electrons

The atomic number is equal to the number of protons in an element.

The oxidation number and valence electrons can vary depending on whether it is an ion or not. The number of neutrons can also vary depending on what isotope the atom is. However, the number of protons cannot be changed without changing the identity of the base atom itself.

The answer is (a) atomic number

Atomic number = no. of protons = no. of electrons