Final answer:
Nitrogen and helium are classified as nonflammable gases, whereas oxygen supports combustion and hydrogen is extremely flammable.
Step-by-step explanation:
The gases that are classified as nonflammable are nitrogen (c) and helium (d). Oxygen (a) is not flammable itself but is a strong oxidizer and greatly supports combustion.
Hydrogen (b) is highly flammable and a potent energy source used in fuel cells.
When it comes to safety, non-reactive and non-flammable gases like helium are preferred for applications where there is a risk of fire, such as in balloons or as coolants in MRI machines, due to their lack of reactivity and non-flammability.