For an elements W, X, Y and Z contain 8, 11, 9 and 17 protons per atom respectively. The element which cannot form an anion is most likely to be

Here, W, X, Y and Z are chemical elements.

The numbers 8, 11, 9 and 17 are the atomic numbers of above four elements. Therefore, the names of elements are oxygen (O), sodium (Na), Fluorine (F) and Chlorine (Cl) respectively.

The element X with atomic number 11 is sodium and it is a metal.

Hence, it cannot form anion.