Which one of the following is not electronic configuration of a noble gas? 

The correct answer is 2, 8,18, 2 i.e option 2

Key Points

•  2, 8, 18, 2. This is not the electron configuration of a noble gas.
• Noble gases, found in Group 18 (VIII A) of the periodic table, are characterized by their completely filled outer electron shells, giving them stability.
• These gases include helium (2), neon (2,8), argon (2,8,8), krypton (2,8,18,8), and xenon (2,8,18,18,8) among others. 4
• The electron configuration of a noble gas always ends with a completed outer shell - typically matching the pattern of 2 or 8 electrons.
• The configuration 2,8,18,2 does not meet this criterion because the last energy level contains 2 electrons, not a complete 8.

Additional Information 
2, 8, 18, 8 (Option 1):

• This is the electronic configuration of Krypton (Kr), a noble gas.
• Beyond its noble gas-status, Krypton is used in certain types of photographic flashes and is mixed with argon in energy-efficient fluorescent lamps. It's also used in some types of lasers that are used for eye surgery.

2, 8 (Option 3):

• This is the electronic configuration of Neon (Ne), a noble gas.
• Aside from being a noble gas, Neon is widely known for its applications in advertising signs. When electricity is passed through neon gas, it gives off a brilliant red-orange color.

2, 8, 18, 18, 8 (Option 4):

• This is the electronic configuration of Xenon (Xe), a noble gas.
•  Xenon, in addition to being a noble gas, is used in certain types of lamps and flashbulbs due to its ability to produce light when excited by an electric discharge.

Conclusion:-

So the 2, 8,18, 2 is not electronic configuration of a noble gas