In a bipolar junction transistor, the correct answer is option 4 - the base has the least concentration of impurity.
A bipolar junction transistor consists of three regions - the emitter, the base, and the collector - which are doped with impurities to create either p-type or n-type semiconductor material.
Option 1 states that all three regions have equal concentrations of impurity. This is incorrect because in a transistor, the concentrations of impurity in each region are different.
Option 2 states that the emitter has the least concentration of impurity. This is not true because the emitter is heavily doped to have a high concentration of impurity in order to provide a large number of majority carriers.
Option 3 states that the collector has the least concentration of impurity. This is also incorrect as the collector is lightly doped to allow for the collection of majority carriers from the base region.
Option 4 correctly states that the base has the least concentration of impurity. The base region is lightly doped to allow for the control of current flow between the emitter and collector regions.
Therefore, option 4 is the correct answer.