To convert an intrinsic semiconductor (such as Silicon or Germanium) into an n-type extrinsic semiconductor, it must be doped with a pentavalent impurity (an element from Group 15 of the periodic table) which provides extra free electrons as majority charge carriers. Phosphorous (P) is a pentavalent element, making it the correct choice. In contrast, Aluminium (Al) is a trivalent impurity which would create a p-type semiconductor, Germanium (Ge) is a Group 14 intrinsic semiconductor itself, and Silver (Ag) is a transition metal.