When carbon-14 decays, one of its neutrons changes to a proton. What is produced by carbon gaining a proton and why? Because carbon-14 has gained a neutron, there is now an extra neutron, which makes a carbon atom with an atomic mass of 15.

Because carbon has lost a proton, there are now 6 protons which makes a carbon atom with an atomic mass of 14.

Because carbon-14 has lost a neutron, there are now 5 neutrons which makes a carbon molecule with an atomic mass of 13.

Because carbon-14 has gained a proton, there are now 7 protons which makes a nitrogen atom with an atomic mass of 14.