Which type of dentin is formed before the completion of the apical foramen and constitutes the bulk of the tooth?

Primary dentin is the type of dentin that is formed before the completion of the apical foramen and represents the major portion of the tooth's structure. It is laid down during the development of the tooth and contributes to the overall shape and size of the tooth as it develops. Primary dentin is formed by odontoblasts and is characterized by a regular pattern of tubules, which are critical for the nutrition and health of the dental pulp.

In contrast, secondary dentin is formed after the completion of the toot's development and continues to be produced throughout life but at a much slower rate. Tertiary dentin, also known as reparative dentin, is formed in response to stimuli such as caries or trauma and is laid down in a more irregular fashion. Peritubular dentin refers to the dentin that surrounds the dentinal tubules and is not a main category of dentin formation like the others.

In summary, primary dentin's role in the overall structure and bulk of the tooth is essential during early development, making it the correct answer in this context.