Diagram A : This shows the strong interaction . A proton and neutron are attracted together by the exchange of a neutral pion.
Diagram B : This shows the electromagnetic interaction. Two electrons repel each other by the exchange of a virtual photon.
Diagram C : This represents beta minus decay. A neutron decays due to the weak interaction into a proton, an electron and an anti electron neutrino
Diagram D : This represents beta plus decay. A proton decays into a neutron, a positron and an electron neutrino