Which of the following materials is most commonly used in direct pulp capping?

Calcium hydroxide is the most commonly used material for direct pulp capping due to its biocompatibility and ability to promote the formation of secondary dentin. This material acts as a protective barrier over the exposed pulp, creating a conducive environment for the pulp to heal. Calcium hydroxide has inherent antibacterial properties and is alkaline, which can help neutralize any acidity in the dental environment, enhancing its ability to support pulp vitality.

When applied directly to the pulp, calcium hydroxide stimulates the reparative capacity of the dental pulp cells, encouraging the formation of a dentin bridge over the exposed area. This is crucial in maintaining the health of the tooth and can potentially help avoid more invasive procedures, such as root canals, if the pulp remains viable. The other materials mentioned, such as composite resin, glass ionomer cement, and amalgam, may serve other purposes in restorative dentistry, but they do not possess the same specific benefits for direct pulp capping as calcium hydroxide does.