Which of the following dopants is used in germanium to form n-type semiconductor?

To form an n-type semiconductor from germanium, a pentavalent impurity with five valence electrons is required. Such impurities donate an extra electron that becomes a conduction electron, making electrons the majority charge carriers.

Among the options: As (Arsenic) is Group 15 with five valence electrons and creates n-type semiconductor; B (Boron), In (Indium), and Ga (Gallium) are all Group 13 elements with three valence electrons, creating hole-dominated p-type semiconductors.

The correct dopant for n-type germanium is therefore $\boxed{A}$.