Concept:An N-P-N transistor is a thin p-type layer (the base) sandwiched between two n-type layers (emitter and collector).Explanation:The middle letter of N-P-N names the base, so the base is p-type. It is doped with a trivalent impurity, so its majority carriers are holes, not electrons. It is not undoped.Answer:Option A (is a p-type semiconductor)