Four 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 :

W = 8 protons,  X = 11 protons

Y = 9 protons, Z = 17 protons

Based on atomic numbers (no. of protons), the elements W,Y, Z  are non metals (O, F and Cl). All they are form anion by gaining one or more electrons. But the element “X” is a metal (Sodium, Na) with the atomic number, Z=11.