• question_answer According  to Aufbau's principle, which of the three $4d,\,5p$   and $5s$will be filled with electrons first           [MADT Bihar 1984] A)                 $4d$ B)                 $5p$ C)                 $5s$ D)                 $4d$ and $5s$ will be filled simultaneously

According to the Aufbau?s principle, electron will be first enters in those orbital which have least energy. So decreasing order of energy is $5p\,>\,4d>\,5s.$