Which of the following sub-shell will be fifth by electron after the orbital of the third principal shell is completely filled ?

Options

A. `4s`
B. `4f`
C. `4d`
D. `4p`

Explanation

To solve the question of which sub-shell will be filled with the fifth electron after the orbitals of the third principal shell are completely filled, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Electron Configuration**:
The electron configuration of an atom shows how electrons are distributed among the various atomic orbitals. The order of filling is determined by the Aufbau principle.

2. **Identify the Filling Order**:
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