The correct answer is option 'A': the emitter-base junction is forward biased and the base-collector junction is reverse biased.

Explanation:
To understand why option 'A' is the correct answer, let's dive into the working principle of a transistor amplifier.

Transistor Amplifier:
A transistor is a three-terminal device that can be used as an amplifier to increase the amplitude of an electrical signal. It consists of three layers of semiconductor material: the emitter, base, and collector.

Emitter-Base Junction:
The emitter-base junction is a forward-biased junction. This means that the emitter terminal is connected to a positive voltage source, while the base terminal is connected to a negative voltage source (in relation to the emitter). This forward biasing allows the flow of majority charge carriers (electrons for NPN transistor) from the emitter to the base.

Base-Collector Junction:
The base-collector junction is a reverse-biased junction. This means that the base terminal is connected to a positive voltage source, while the collector terminal is connected to a negative voltage source (in relation to the base). This reverse biasing prevents the flow of majority charge carriers (electrons for NPN transistor) from the collector to the base.

Working Principle:
When a small input signal is applied to the base-emitter junction, it controls the flow of current through the emitter-base junction. This controlled current results in a larger current flowing through the collector-emitter junction, making the transistor act as an amplifier.

Here's a breakdown of the working principle:

1. Input Signal: The small input signal is applied to the base-emitter junction. This signal can be an AC voltage or current.

2. Forward Biasing: The forward-biased emitter-base junction allows the input signal to control the flow of current through the base-emitter junction.

3. Current Amplification: The controlled current through the base-emitter junction causes a larger current to flow through the collector-emitter junction. This current amplification is the key characteristic of a transistor amplifier.

4. Output Signal: The amplified output signal is obtained at the collector terminal. It is a larger version of the input signal, with higher amplitude.

Therefore, to use a transistor as an amplifier, the emitter-base junction must be forward biased, allowing the input signal to control the current flow. The base-collector junction is reverse biased to prevent the flow of current from the collector to the base.

In conclusion, option 'A' is the correct answer as it correctly describes the biasing conditions required for a transistor amplifier.