Answer: Nitrogen
Explanation: Out of silicon, nitrogen and aluminum, Nitrogen can be used effectively and can replace Phosphorous in solid solution .
Because Nitrogen and Phosphorous belong to the same family of Pnictogens that is Group 15.
They are placed in the same group due to the similarities in the valence electrons of the outermost shell and due to the similarities in the chemical and physical properties.
Outermost electron configuration of Group 15 elements is
ns2 np3
where n = electron shell.