If the atomic number of an element remains the same but the number of neutrons has been increased, which factor has been changed in that element?

the number of electrons

the chemical symbol

the number of protons

the mass number

I think the correct answer from the choice listed above is the last option. If the atomic number of an element remains the same but the number of neutrons has been increased, then the mass number has been changed. The atomic number is also known as the proton number, and if that changes then the name of the element would also change.