The formation of ionic bonds is driven by the difference in electronegativities of the atoms. You can find electronegativiies in a table. In fact the electronegativities of thosw four compounds are N: 3.04, O: 3.44, P: 2.19, C: 2.55 and K: 0.82. That tells you that the bigger difference is with K, then K will most likely form an ionic compound with N. And that is the answer. The position in the periodic tables, and even the kind of elements involved, can guide you about this. O, P and C are neighbors of N, so can predict that they have similar electronegativities, while K is far from N in the periodic table. A good indication of substantial difference in electronegativities. Also, K is a metal while the other elements are non metals. Non metals do not form ionic compound among them, and many metals form ionic compounds with nonmetals.