Which of the following pairs of 'number - composition' is correct?

I. Atomic number - number of protons

II. Mas number - Sum of number of neutrons and protons

Both I and II

The correct answer is option 3. Both I and II.

The correct pair of 'number - composition' is Both I and II

I. Atomic Number:  The atomic number is indeed the number of protons in an atom. It uniquely identifies an element and determines its position in the periodic table.

II. Mass Number: The mass number is the sum of the number of protons and neutrons in the nucleus of an atom. It represents the total nucleons (protons + neutrons) in an atom.