Identify the missing particle in the reaction.
This is about combining subatomic particles and balancing out energy and mass on either side of the reaction. The first element in the reaction is a proton and the second reactant is an antiproton. Among other things, it has the opposite charge of a proton. But for our purposes, the important point is that they annihilate one another, with energy remaining, but no mass.
As we look at the first product of this reaction, we see it’s a neutron, which does have mass. So whatever particle goes in the blank, whatever particle is missing from this reaction, must combine with the neutron to annihilate it, removing all the mass from the product’s side of this reaction. Just as protons have antiprotons, neutrons have antineutrons; that is the missing particle in this reaction.