In the periodic table, identify the element X on its average atomic mass

Potassium

Explanation:

To solve this question we need to find, as first, the average atomic mass of the element knowing that is defined as the sum of the masses of each isotope times their abundances. For the element X:

Average atomic mass:

38*0.0967 + 39*0.7868 + 40*0.1134 + 41*0.0031

= 39.02g/mol

The atom that has the average atomic mass closest to 39.02g/mol is Potassium, K = 39.10g/mol.