I would have said, at the very least d-rank, but yeah this is bullshit, since that means that the guild's system is obsolete, since their most capable adventurers can be 2-3 ranks lower than other random ones. The whole point of the ranking is to have a record of the adventurer's capabilities, to be able to assign them to missions of their levels. So forcing capable people of staying at lower ranks for no good reasons is counter productive...