In beta-plus decay (β+ decay), a proton in the nucleus is converted into a neutron, and a positron (the antimatter counterpart of an electron) and a neutrino are emitted. This process decreases the atomic number by one (since there is one less proton) while the mass number remains the same (the total number of nucleons, protons plus neutrons, does not change).
Therefore, the correct response is:
The mass number remains the same, and the atomic number decreases by one.