The emitter has the greatest concentration of impurity so that the large number of carriers traverses the path to the collector via base and hence contributes to desire current. Some carriers (holes or electrons) while traversing this path get recombine with carriers (electrons or holes) present in base and thus current will decrease. If the base has impurity concentration greater than the collector, more carriers get recombine in the base and less carriers will go to the collector thus contributing current less than desired. Hence, the base should have impurity concentration less than the impurity concentration in the collector so that there will be much less recombination in base and it contributes to desire current. Therefore, the base has least impurity concentration to keep recombination much less. The collector is moderately impure. So the correct answer is option C.