Q.In a transistor (A) concentration of impurities in the collector is minimum
(B) concentration of impurities in the emitter is minimum
(C) concentration of impurities in the base is minimum
(D) concentrations of impurities in all the three regions are same

A transistor is, at its simplest, a sandwich. But it is a very special sandwich, made of semiconductor materials that have been treated — or doped — to have either an excess of electrons (n-type) or a deficit of electrons, which we call "holes" (p-type).

The intuition starts with a single p-n junction, which is just a diode. A diode lets current flow in one direction but blocks it in the other. A transistor takes two such junctions and places them back-to-back, sharing one thin middle layer. That shared layer is the key.

If you take a piece of p-type material and put a thin slice of n-type material in the middle, you get an npn transistor. If you reverse the order — n-type on the outside, p-type in the middle — you get a pnp transistor. The three regions are given names based on their role in the circuit, not just their position.

The outer regions are called the emitter and the collector. The thin middle region is the base.

The emitter is heavily doped. Its job is to emit charge carriers (electrons for npn, holes for pnp) into the base. The collector is moderately doped and physically larger than the emitter. Its job is to collect those carriers after they have crossed the base.

So the precise structure is this: three distinct doped semiconductor regions arranged in a sequence — either npn or pnp — with the middle region (the base) being very thin and lightly doped, and the two outer regions (emitter and collector) being more heavily doped, with the emitter being the most heavily doped of all.