The mass number of an atom is the total number of protons and neutrons in its nucleus.

$$\text{Mass number} = \text{number of protons} + \text{number of neutrons}$$

The atom has:

• $15$ protons
• $20$ neutrons
• $15$ electrons

Electrons do not contribute to the mass number, because their mass is very small compared with protons and neutrons.

So:

$$\text{Mass number} = 15 + 20$$ $$\text{Mass number} = 35$$

Therefore, the correct answer is:

C) 35