Question

why is collector current slightly less than emitter current in a transistor?​

Answer 1

Some of the electrons combine with the majority charge carrier holes present in the base and most of the electrons reach the collector, crossing the collector-base junction. This implies that collector current is always less than the emitter current due to the reason (b).

Answer 2

Some of the electrons combine with the majority charge carrier holes present in the base and most of electrons reach the collector , crossing the collector box junction. This implies that collector current is less than emitter current due to the reason.