Which part of a transistor is heavily doped and emits majority carriers, either electrons or holes? 

The correct option is 2

Concept:

The transistor consists of three main regions i.e. Emitter, Base, and Collector.

Emitter (E):

• It provides majority charge carriers by which current flows in the transistor.
• Therefore the emitter semiconductor is heavily doped.
• The emitter is always forward-biased since its function is to provide charge carriers.

Base (B):

• The base region is thin and lightly doped. This reduces the base transit time which prevents the charge from the emitter from recombining at the base and passing directly to the collector (for maximum efficiency).
• It provides proper interaction between the emitter and collector

Collector (C):

• The size of the collector region is larger than the two other regions and it is moderately doped.
• The main purpose of the collector is to collect the majority of charge carriers from the emitter. 

NPN transistors:

It is formed by sandwiching a thin layer of P-type semiconductor between two N-type semiconductors as shown:

PNP transistor:

It is formed by sandwiching a thin layer of N-type semiconductor between two P-type semiconductor as shown: