K- mesons are made up of an anti-up quark and a strange quark. They decay via the weak interaction. At normal energies, the weak interaction will change the strange quark into an up quark, a lepton and an antineutrino.
So we're left with an up quark and an anti-up quark, which together form a neutral pi0 as well as a lepton and an antineutrino. The lepton can either be an electron or a muon.
The best way to remember it is by conservation of charge and conservation of particle minus antiparticle number. The negatively charged K- goes to a neutral pi0 so the lepton has to be negatively charge. But since you have an extra particle you need an extra antiparticle for conservation, and it must be neutral too, so the charge balance is unaffected. So you get an antineutrino.
The in depth explanation for precisely why this happens is waaaaay beyond A level, so not sure why they expect you to remember.