Question

Which of the following is a reasonable ground-state electron configuration?

O 1s21p5
O1s22s22p8
O 1s22s22d4
O 1s?2s22p6

Answer 1

the last one (if we consider the typo ? as 2)

Step-by-step explanation:

p orbital can hold 6 electrons

1st orbit only has 's' orbital

2nd orbit only has 's' & 'p' orbital, no 'd' orbital