Which of the following is not a noble gas?  

Radium 

The correct answer is option 3. Radium.

Among the options provided, radium is not a noble gas. Radium (Ra) is a highly radioactive element belonging to Group 2 of the periodic table, known as the alkaline earth metals. It is a solid metal at room temperature and does not exhibit the chemical properties associated with noble gases.

On the other hand, neon (Ne), argon (Ar), and radon (Rn) are all noble gases. They belong to Group 18 of the periodic table and are characterized by their extremely low reactivity due to their stable electronic configurations.

So, the correct answer is Radium.