One company developed what it called a 'technology shelf', created by a few enginerrs, on which [was placed/were placed/placed] possible techinical solutions that other teams might use in the future.
According to the guildeline, it is said that 'were placed' was grammatically correct, because the subject of the sentence is 'possible technical solutions' as inversion happened in the relative clause.
However, I feel somewhat awkward about the it, and still can't understand that the inversion could happen in the relative clause with preposition.
If so, why not always does the inversion happen in relative clause with prepostion?