The correct answer is option 'C': The base of an NPN transistor must be positive with respect to the emitter. Let's explore why this is the case in more detail.

NPN Transistor Operation:
An NPN transistor consists of three layers of semiconductor material: the emitter, base, and collector. The base is lightly doped compared to the emitter and collector, which are heavily doped. The transistor operates in three different modes: cutoff, active, and saturation.

1. Cutoff Mode:
In the cutoff mode, the transistor is turned off, and no current flows between the collector and emitter. The base-emitter junction is reverse biased, which means the base voltage is lower than the emitter voltage.

2. Active Mode:
In the active mode, the transistor acts as an amplifier. The base-emitter junction is forward biased, which means the base voltage is higher than the emitter voltage. This biasing allows a small current to flow from the emitter to the base, which controls a larger current flow from the collector to the emitter.

3. Saturation Mode:
In the saturation mode, the transistor is turned on fully, and there is maximum current flow between the collector and emitter. The base-emitter junction is still forward biased.

Base-Emitter Biasing:
To operate the NPN transistor in the active mode, the base-emitter junction must be forward biased. This means that the base must be positive with respect to the emitter. The positive voltage at the base attracts the electrons from the emitter towards the base region, allowing current to flow from the emitter to the base.

If the base is negative with respect to the emitter, the base-emitter junction will be reverse biased, and no current will flow through the transistor. Therefore, option 'C' is the correct answer.

Conclusion:
In conclusion, the base of an NPN transistor must be positive with respect to the emitter for it to operate in the active mode. This forward biasing of the base-emitter junction allows current to flow through the transistor, enabling its amplification function.