In beta-plus decay (or positron emission), a proton in the nucleus is transformed into a neutron, and a positron (the antimatter counterpart of an electron) is emitted.
As a result:
- The mass number remains the same, because the total number of nucleons (protons + neutrons) does not change.
- The atomic number decreases by one, as the nucleus loses a proton.
Therefore, the correct response is:
The mass number remains the same, and the atomic number decreases by one.