A npn transistor conducts when the base
both collector and emitter are positive with respect to
collector is positive and emitter is negative with respect to the base
Collector is positive and emitter is at the same potential as the base
both collector and emitter are negative with respect to the base
When the collector is positive and emitter is negative w.r.t base, it causes the forward biasing for each junction, which causes conduction of current.