Which of the following substances is classified as an ionic compound?

Sodium hydroxide is classified as an ionic compound because it is formed from the electrostatic attraction between positively charged sodium ions (Na⁺) and negatively charged hydroxide ions (OH⁻). This results in the formation of a stable ionic bond. Ionic compounds are typically composed of metals and non-metals, with the metal losing electrons to form positive ions while the non-metal gains electrons to become negative ions.

In contrast, the other substances listed are not ionic compounds. Methane and carbon dioxide are covalent compounds, formed by sharing electrons between atoms rather than transferring them. Water, while polar and possessing a significant dipole moment due to the difference in electronegativity between hydrogen and oxygen, is also a covalent compound as it consists of polar covalent bonds rather than ionic bonds. Therefore, sodium hydroxide stands out as the only ionic compound among the options provided.