* Ionic compounds form when a metal (typically a cation) and a nonmetal (typically an anion) transfer electrons.

* Potassium (K) is a metal and will readily lose one electron to form a +1 cation (K+).

Let's analyze the options:

* a. Na (Sodium): Sodium is also a metal and will form a +1 cation (Na+). They would likely form a metallic bond, not an ionic bond.

* b. Ca (Calcium): Calcium is a metal and will form a +2 cation (Ca+2). While it could react with potassium, it's not the most likely option.

* c. C (Carbon): Carbon is a nonmetal. It's more likely to form covalent bonds, sharing electrons, rather than ionic bonds.

* d. O (Oxygen): Oxygen is a nonmetal and will readily gain two electrons to form a -2 anion (O-2).

Therefore, the answer is (d) O (Oxygen).

Potassium (K+) and Oxygen (O-2) will readily form an ionic compound called Potassium Oxide (K2O).