This particle is found in the nucleus and adds mass to the nucleus, but changing its number does not change what the element is. this particle is the a. proton. b. photon. c. neutron. d. electron.

Adding neutron to the nucleus creates new isotopes of that element. Mass of the nucleus will increase if you add neutron to it but element will remain the same. If you add proton to the nucleus, mass of the nucleus will change but also element will also change. That is, atom of the element will change. 

Ithink the answer would be a