Identify the noble gases among the options

Noble gases are Group 18 elements: $\text{He, Ne, Ar, Kr, Xe, Rn}$

From the options:

• A: Ne $\rightarrow$ Noble gas
• B: He $\rightarrow$ Noble gas
• C: Kr $\rightarrow$ Noble gas
• D: Ra $\rightarrow$ Not a noble gas; it is radium, an alkaline earth metal.

Check which noble gases occur in the atmosphere

• Neon (Ne) is present in trace amounts in air.
• Helium (He) is present in very small amounts in the atmosphere.
• Krypton (Kr) is also present in trace amounts in air.

Interpret the intended question

Since option D is written as Ra, it is not a noble gas at all. In such questions, this is almost certainly a misprint for Rn (radon), which is a noble gas.

Among noble gases, radon (Rn) is generally considered not to occur in the atmosphere in the usual atmospheric composition tables because it is radioactive and present only locally/in negligible transient amounts.

Choose the best answer from the given options

Because the stored option is D, and the intended noble gas is almost certainly Rn, the expected answer is: $\boxed{D}$

Important note

As written, Ra is not a noble gas, so the question contains a typo. Chemically, the correct intended species is almost certainly Rn.