Answer:

D. 4f

Explanation:

According to the Auf bau Principle (remove space between f and b), the electron configuration is filled in this order.

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, ...

The underlined configuration is the given, and since the clue word is "before", we can conclude that 4f is filled before 5d.

Answer:

Required Answer :-

D. 4f ☑️

Use,

Aur bau principle of finding orbital

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p,

We may observe that

5d just come before 4f