Doping in Semiconductors:
Doping is the process of intentionally adding impurities to a semiconductor material in order to change its electrical properties. By doping a semiconductor, we can create either n-type or p-type regions within the material.

n-type Semiconductors:
In n-type semiconductors, the majority charge carriers are electrons. This is achieved by introducing impurities with extra electrons into the semiconductor crystal lattice. These impurities are called donor impurities.

Doping Germanium:
Germanium is a Group 14 element in the periodic table and has 4 valence electrons. In order to create an n-type semiconductor, we need to introduce impurities that have 5 valence electrons, which will provide an extra electron to the crystal lattice.

Arsenic:
Arsenic is a Group 15 element and has 5 valence electrons. When arsenic is introduced as a dopant in germanium, it replaces some of the germanium atoms in the crystal lattice. As arsenic has one more valence electron than germanium, the extra electron becomes delocalized and contributes to the conduction process, resulting in the formation of n-type germanium.

Aluminium:
Aluminium is a Group 13 element and has 3 valence electrons. When aluminium is introduced as a dopant in germanium, it replaces some of the germanium atoms in the crystal lattice. However, since aluminium has fewer valence electrons than germanium, it creates a "hole" in the crystal lattice instead of providing an extra electron. This leads to the formation of p-type germanium.

Boron:
Boron is a Group 13 element and has 3 valence electrons. When boron is introduced as a dopant in germanium, it replaces some of the germanium atoms in the crystal lattice. Similar to aluminium, boron has fewer valence electrons than germanium, resulting in the formation of p-type germanium.

Gallium:
Gallium is a Group 13 element and has 3 valence electrons. Like aluminium and boron, when gallium is introduced as a dopant in germanium, it replaces some of the germanium atoms in the crystal lattice. Therefore, gallium also creates a hole in the crystal lattice, leading to the formation of p-type germanium.

Conclusion:
To achieve n-type germanium, we need to introduce impurities with 5 valence electrons. Among the options provided, arsenic is the correct choice as it has 5 valence electrons and can provide the extra electron needed for n-type doping.