In active mode of a BJT transistor, the emitter-base junction is kept forward biased while the collector-base junction is kept reverse biased. The transistor acts as an amplifier in this mode. Hence, option (B) is the correct answer. 

Collector-Base junction in BJT:
The Collector-Base (CB) junction is one of the three junctions in a Bipolar Junction Transistor (BJT). It is formed between the collector and the base regions of the transistor. The CB junction plays a crucial role in the operation of the BJT.

Operating modes of BJT:
There are three operating modes of a BJT - active mode, saturation mode, and cutoff mode. In the active mode, the BJT operates as an amplifier. To ensure that the BJT operates in the active mode, certain conditions need to be met.

Role of Collector-Base junction:
The CB junction is reverse-biased in all operating modes of the BJT. However, to operate the BJT in the active mode, the CB junction must be reverse-biased with a sufficient voltage.

Explanation:
In the active mode, the CB junction is reverse-biased. This means that the collector terminal is at a higher voltage potential than the base terminal. This reverse bias creates a depletion region in the CB junction, which prevents the flow of majority carriers (electrons in an NPN transistor or holes in a PNP transistor) across the junction.

When the collector-base junction is reverse-biased, it prevents the majority carriers from entering the depletion region. This allows the base current to control the transistor's operation. The base current causes a proportional amount of collector current to flow, resulting in amplification.

If the CB junction is forward-biased, it would allow the majority carriers to flow freely across the junction, bypassing the base current control. This would lead to the BJT operating in either saturation mode or cutoff mode, depending on the biasing conditions.

Therefore, to operate the BJT in the active mode, the CB junction must be reverse-biased. This ensures that the base current controls the transistor's operation, allowing it to function as an amplifier.

In conclusion, the correct answer is option 'B' - the Collector-Base junction must be reverse-biased for the BJT to operate in active mode.