Which elements combine by forming an ionic bond
a) sodium and potassium
b) sodium and oxygen
c) carbon and oxygen
d) carbon and sulfur

B) Sodium and oxygen

Explanation:

Ionic bond: This type of bond is formed when there is a complete transfer of electrons from one element to another element. Ionic bond is the bond which is formed between the two oppositely charged ions and are held by the strong electrostatic attraction. This is formed between the atoms which have high differing electronegativities. In this bonding one element is always a metal and another is a non-metal.

Thus,

Out of the options, B involves a metal and a nonmetal which is sodium and oxygen respectively. Thus, they can form a ionic bond on combining.

A) represents 2 metals, C) and D) represents 2 nonmetals and thus, they cannot form an ionic bond.