Assertion
A transistor amplifier in common emitter configuration has low input impedance
Reason
The base to emitter region is forward biased
A) If both the assertion and reason are true and reason is a true explanation of the assertion.
B) If both the assertion and reason are true but the reason is true the correct explanation of the assertion.
C) If the assertion is true but the reason is false.
D) If both the assertion and reason are false.

Hint: A transistor which is configured as common emitter, common base and common collector. Emitter is always forward biased with respect to base to supply majority charge carriers to the base. Collector terminal is always reverse biased with respect to the base to remove the charge carriers from the base collector junction.

Complete step-by-step solution:
An amplifier is a device which is used for increasing the amplitude of input signals.in an amplifier there are two input terminals for connecting input and two output terminals for connecting load. In a common emitter configuration, base is one of the input terminals and collector is one of the output terminals.

For proper functioning of a transistor, the emitter-base junction is forward biased and collector-base junction must be reverse –biased.
The input impedance of common emitter configuration with ${{V}_{CE}}$ as constant is given as:
${{Z}_{E}}=\dfrac{\Delta {{V}_{BE}}}{\Delta {{I}_{B}}}$
Where ${{V}_{BE}}$ = Voltage across base and emitter
              $\Delta {{I}_{B}}$ = Base current
Base current is of order of few microamperes so the input impedance is very low
So the correct option is A.

Note: In common emitter circuits its current, voltage and power gains are quite high and the ratio of output impedance and input impedance are quite moderate. As it provides high voltage gain and moderate current gain. Common emitter circuits are well suited for voltage amplification for low frequencies.