Explanation: For an NPN silicon transistor to allow current to flow from emitter to collector, its base-emitter (B-E) junction must be forward-biased. This means the base must be made positive with respect to the emitter. Silicon diodes and transistor junctions have a characteristic "cut-in" or "threshold" voltage that must be overcome for significant current to flow. For silicon, this voltage is approximately 0.7 volts. Below this voltage, the junction acts largely as an open circuit, and the transistor remains in cutoff, preventing collector current. Once the base voltage reaches about 0.7V positive relative to the emitter, the junction becomes sufficiently forward-biased, allowing a small base current to control a much larger emitter-collector current. Options A (0.4 volts) is generally too low for silicon, though it might apply to germanium transistors. Options B and D (negative voltage) would reverse-bias the base-emitter junction, keeping the transistor in cutoff and preventing current flow.