Which of the following is NOT a noble gas?

The correct answer identifies an option that is not a noble gas. In the context of the periodic table, noble gases are a group of elements found in Group 18, which are characterized by their full valence electron shell, resulting in a lack of chemical reactivity under standard conditions. The noble gases include helium, neon, argon, krypton, xenon, and radon.

Krypton, despite being listed as an option, is indeed a noble gas. Therefore, it would not be the correct answer as the question asks for an element that does not belong to this group.

In identifying a choice that is not a noble gas, one would have to recognize elements outside of Group 18, such as those in Groups 1 to 17, which include reactive nonmetals, transition metals, and metalloids. It's important to familiarize yourself not only with noble gases but also with the layout of the periodic table and the properties that distinguish noble gases from other elements.