You have made mistake in SIM1, that is you missed few state pairs in step 0. According to SIM1, two states are in a relation if they agree on common variables.
(s1, s2) ∈ σ implies L1(s1) ∩ V2 = L2(s2) ∩ V1
So in this question the common variable is 'a' and the states in a relation are those that agree upon the variable 'a'.
Eg: (Q3, S0) is a state pair that satisfies SIM1.