Which of the following elements is NOT a noble gas?

1. A

   Argon

   Argon. Option A claims that Argon is a noble gas, which is factually correct because it belongs to Group 18 of the periodic table.

2. B

   Krypton

   Krypton. Option B states that Krypton is a noble gas, which is correct as it is a naturally occurring unreactive noble gas in Group 18.

3. C

   Radon

   Radon. Option C posits that Radon is a noble gas, which is also correct since it is a radioactive element classified under Group 18.

4. D

   Actinium

   Actinium. Option D asserts that Actinium is a noble gas. This is incorrect because Actinium is a radioactive metallic element belonging to the actinide series, making this option the correct answer to the exclusion question.

Summary. Official key is (d) Actinium. The question asks to identify the element that is NOT a noble gas among the given choices. Argon, Krypton, and Radon are all members of Group 18, commonly known as the noble gases. Actinium, on the other hand, is a transition metal within the actinide series. Therefore, Actinium is correctly identified as the element that does not belong to the noble gas family.