The order of filling of electrons in the orbitals of an atom is determined by the Aufbau principle, which states that electrons fill the lowest energy level orbitals first before moving to higher energy levels. The correct order of filling for the given orbitals is:

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

Explanation:

1. 1s orbital: The 1s orbital is the first energy level and can hold a maximum of 2 electrons.

2. 2s and 2p orbitals: The 2s orbital is the second energy level and can hold a maximum of 2 electrons. The 2p orbital is also in the second energy level and can hold a maximum of 6 electrons (2 in each of the three p orbitals).

3. 3s and 3p orbitals: The 3s orbital is in the third energy level and can hold a maximum of 2 electrons. The 3p orbital is also in the third energy level and can hold a maximum of 6 electrons (2 in each of the three p orbitals).

4. 4s, 3d, and 4p orbitals: The 4s orbital is in the fourth energy level and can hold a maximum of 2 electrons. The 3d orbital is also in the fourth energy level and can hold a maximum of 10 electrons (2 in each of the five d orbitals). The 4p orbital is also in the fourth energy level and can hold a maximum of 6 electrons (2 in each of the three p orbitals).

5. 5s, 4d, and 5p orbitals: The 5s orbital is in the fifth energy level and can hold a maximum of 2 electrons. The 4d orbital is also in the fifth energy level and can hold a maximum of 10 electrons (2 in each of the five d orbitals). The 5p orbital is also in the fifth energy level and can hold a maximum of 6 electrons (2 in each of the three p orbitals).

Therefore, the correct order of filling for the given orbitals is 4s, 3d, 4p, 5s, 4d, which is option 'B'.