A group 14 element was converted into an n-type semiconductor by doping it with:

Phosphorus

The correct answer is option 4.  Phosphorus.

Group 14 elements have 4 valence electrons. In their pure state, they form covalent bonds with their neighbors, filling all their outer shell orbitals. This results in a semiconductor with a band gap, meaning it doesn't readily conduct electricity.

N-type semiconductors conduct electricity due to the presence of excess electrons. This is achieved by doping, which introduces impurities with 5 valence electrons (from group 15) into the crystal lattice.

When a group 15 element like phosphorus is added, it substitutes for a group 14 atom. Due to having one more electron than the host atom, this extra electron becomes loosely bound and easily excited into the conduction band. This creates a mobile charge carrier, leading to n-type conduction.

Aluminium (group 13) has 3 valence electrons, silicon (group 14) has the same number of electrons as the host atom, and gallium (group 13) also has 3 valence electrons. Doping with these elements wouldn't create the necessary excess electrons for n-type conductivity.

Therefore, only phosphorus (group 15) among the given options fulfills the requirement for making the group 14 element an n-type semiconductor.