To form n-type semiconductor from a pure semiconductor, the impurity to be added is

Arsenic

The correct answer is Option (4) → Arsenic

n-type Semiconductor: Formed by doping a pure (intrinsic) semiconductor like silicon or germanium with a pentavalent impurity (having 5 valence electrons).

Pentavalent elements donate extra electrons, increasing the number of free electrons (negative charge carriers).

Arsenic (As) is a pentavalent impurity. Others listed (Al, B, In) are trivalent and used for p-type semiconductors.