can you tell me how the beta-plus decay process may be understood (i) at the level of individual nucleons and (ii) at the level of quarks, by writing a decay reaction in each case.

Answer 1

See above.

Answer 2

At the level of individual nucleons, a neutron is converted into a proton plus a beta particle (high energy electron) and an electron antineutrino.

A proton consists of 2 up and 1 down quark and a neutron consists of 1 up and 2 down quarks.