For something to be mixed with artificial air (oxygen/nitrogen mix), it must be in the form of gas as solids/liquids will hinder breathing.
Based on the Periodic Table, only D is the suitable element to be included in the scuba tank. In this case, the element is actually Helium, which is added " to reduce the proportions of nitrogen and oxygen below those of air* , to allow the gas mix to be breathed safely on deep dives*.
1 Like
Thank you very much! 
