Baryons are not fundamental particles because they are made of smaller pieces (quarks).
A baryon is made from 3 quarks. Baryon number is conserved in an interaction. In other words, if you have a baryon before the interaction, then you need to have a baryon after the interaction.
The odd bit is that an antiparticle like an antiproton has a baryon number of -1. So if a proton and an antiproton interact, the overall baryon number is +1 + (-1) = 0, so the total baryon number after the interaction must be zero as well.
Leptons ARE fundamental particles. They are not made of smaller particles. Lepton number is conserved in an interaction.
Strangeness is conserved in the strong and electromagnetic interactions. Strangeness is strictly not conserved in the weak interaction but the exam keeps coming up with the SPECIAL CASE where you have zero strangeness before and zero strangeness after, so the mark scheme says strangeness is conserved, even though it's a weak interaction.
