Xenon
Explanation:
The average atomic mass of an element is defined as the sum of the masses of its isotopes,each multiplied by their natural abundances.
Option A:
Argon has a average atomic mass of ![39.948 amu](/tpl/images/0386/1717/72e60.png)
Option B:
Xenon has a average atomic mass of ![131.293 amu](/tpl/images/0386/1717/7c68b.png)
Option C:
Krypton has a average atomic mass of ![83.798 amu](/tpl/images/0386/1717/8b434.png)
Option D:
Neon has a average atomic mass of ![20.1797 amu](/tpl/images/0386/1717/d5a73.png)
So,Xenon has the highest average atomic mass.