Identify the defect that develops when the ratio of number of cations to anions become different from that indicated by its chemical formula.

The defect that alters the cation-anion ratio from the stoichiometric formula is a nonstoichiometric defect. Schottky defects preserve stoichiometry through balanced vacancies, interstitial defects may or may not affect ratios depending on type, and substitutional impurities do not inherently disrupt the original compound's stoichiometric proportion.

Nonstoichiometric defects explicitly create excess metal (via anion vacancies or interstitial cations) or metal deficiency (via cation vacancies), directly changing the $\frac{\text{cations}}{\text{anions}}$ ratio. These deviations are characteristic of compounds exhibiting variable composition.

The correct answer is $\boxed{D}$.