Final answer:
Calcium Hydroxide is placed in the deepest part of the cavity prep, closest to the pulp, and stimulates dentin formation.
Step-by-step explanation:
The substance placed in the deepest part of the cavity prep, closest to the pulp, and stimulates dentin formation is Calcium Hydroxide.
Calcium hydroxide is commonly used in dentistry as a pulp capping material. It helps to promote the formation of reparative dentin, which is a natural defense mechanism of the tooth against infection and injury.