The correct option is C Emitter-base junction is forward biased, and collector-base junction is reverse biased.
Option (A): The Emitter, Base and Collector regions have different sizes with varying concentrations.
The order of size of these regions are: Base < Emitter < Collector.
The order of the concentration of the regions are:
Base < Collector < Emitter
Hence, these regions are doped differently, and their sizes are also different. Hence, this option is wrong.
The base region of a transistor provides an interaction between the emitter and collector of the transistor and this region is made thin to reduce the recombination of holes and electrons in this region, so that there is appreciable collector current.
Hence, (B) is correct.
Option (C): The emitter region has a high concentration of electrons. So, connecting them across the base will result in their movement to the base and then, the collector. To facilitate the movement of the charge carriers from emitter, the emitter-base junction is forward biased, and the base-collector junction is reverse biased so as to attract these charge carriers towards the collector region.
Hence, (C) is correct and (D) is incorrect.
Therefore, options (B) and (C) are the correct answer.