Answer
1212 MeV.
Work Step by Step
To create the $\eta^o$, the minimum available energy is the rest mass energy of the 3 particles that are produced.
This is 938.3 MeV + 938.3 MeV + 547.3 MeV.
Each of the incident protons has half this energy, so the beam energy is 1212 MeV.
