If a trivalent impurity is mixed in a pure (intrinsic) semiconductor, then it becomes a p− type semiconductor. As given in problem, all boron, gallium and aluminium are trivalent impurities so, they must be doped to Ge (or Si) to make it p− type semiconductor.
The added impurity is very small of the order of 1 atom million atoms of the pure semiconductor. When impurity atoms are incorporated in a pure semiconductor crystal, the electric properties are altered and most of the charge carriers originate from impurity atoms.