Carbon dioxide is made up of Carbon and Oxygen so can be broken down… that eliminates A.
Table Salt (NaCl) is also made of two different elements so can be broken down, eliminating B.
Water (H20) is made of two different elements and can be broken down, eliminating C.
That leaves us with answer D, Helium, which is a singular element. This means it cannot be broken down any further by chemical means. This is the correct answer.
Answer: D. Helium