Which of the following best describes hard deposits on teeth?

Hard deposits on teeth are best described as calculus, which forms through the mineralization of dental plaque when not adequately removed by proper oral hygiene practices. Calculus is often visible as a hard, yellowish or brownish deposit on the teeth. While plaque is a soft, sticky film of bacteria that can be brushed away, calculus requires professional dental intervention for removal due to its hardened state.

This choice correctly identifies that these deposits cannot be removed by routine brushing and flossing but must be addressed by a dental professional using specialized instruments. This distinguishes calculus from other types of deposits, such as soft plaque or stains from food and beverages, which can usually be managed through daily oral hygiene practices.