Answer:- N, I and F.
Explanations:- The only diatomic elements in periodic table are hydrogen, nitrogen, oxygen and halogens(17th group elements).
In the given list nitrogen, iodine and fluorine are the diatomic elements where as magnesium, sulfur, argon and helium are not diatomic.
being diatomic, nitrogen, iodine and fluorine are written as
,
and
.