Which one of the following is an ionic compound?

The correct answer is sodium hydroxide, which is an ionic compound. Ionic compounds are formed when there is a transfer of electrons from one atom to another, typically between metals and nonmetals. In the case of sodium hydroxide, it consists of sodium ions (Na⁺) and hydroxide ions (OH⁻). The sodium atom loses one electron to form a positively charged ion, while the hydroxide group, which is a combination of oxygen and hydrogen, carries a negative charge. This transfer of electrons creates a strong electrostatic attraction between the positively and negatively charged ions, resulting in the formation of NaOH as a solid compound, which is characteristic of ionic substances.

The other options listed do not qualify as ionic compounds. Carbon dioxide, for example, is a covalent compound formed through the sharing of electrons between carbon and oxygen atoms. Petrolatum and mineral oil are both organic compounds primarily made up of hydrocarbons, which are held together by covalent bonds rather than ionic bonds. Thus, the distinct characteristics of sodium hydroxide highlight why it is classified as an ionic compound while the others are not.