Which element on the periodic table possesses the highest electronegativity value, making it the most reactive non-metal?

Electronegativity is the measure of an atom's ability to attract electrons in a chemical bond. On the periodic table, electronegativity generally increases from left to right across a period and decreases from top to bottom down a group.

Fluorine (F) is located in the top right corner of the periodic table (excluding noble gases) and has the highest electronegativity value of all elements.

Non-metals tend to gain electrons to achieve a stable electron configuration. A higher electronegativity means a stronger pull for electrons, making the non-metal more reactive in forming bonds by gaining electrons. Therefore, the element with the highest electronegativity will be the most reactive non-metal.

• A. Helium is a noble gas and is very unreactive, with a very low or undefined electronegativity.
• B. Oxygen is highly electronegative, but less so than fluorine.
• C. Chlorine is electronegative, but less so than oxygen and fluorine.
• D. Fluorine has the highest electronegativity (4.0 on the Pauling scale) and is the most reactive non-metal.

The correct answer is D.

The final answer is $\boxed{\text{D. Fluorine}}$