**Core Concept:**

The question is related to the anatomy and morphology of teeth, specifically the crown morphology of teeth. Teeth are classified into different types based on the shape of their crowns, which is determined by the arrangement of cusps and fissures. The primary grooves (or pit and fissure) extend from the cervical area towards the apex of the tooth.

**Why the Correct Answer is Right:**

The correct answer is D, which states that if the extension of a cavity from a primary groove towards the cusp tip is more than 2/3, it is considered as a deep carious lesion. This is based on the concept of the ratio of the length of the carious lesion to the length of the primary groove. A deep carious lesion extends beyond two-thirds of the length of the primary groove, indicating significant involvement of the tooth structure and potential for pulpal involvement.

**Why Each Wrong Option is Incorrect:**

A) This option incorrectly states that the extension of a cavity should be less than 1/3. However, it is the extension of more than 2/3 that defines a deep carious lesion.

B) Similar to option A, this option is incorrect because it refers to less than 1/3, whereas the correct criterion is more than 2/3.

C) This option is incorrect as it refers to the extension being equal to 1/3, which is not the defining factor for a deep carious lesion.

**Clinical Pearl:**

Understanding the significance of carious lesion extension is crucial in clinical practice. Detecting deep carious lesions early allows for timely intervention, such as restorative treatment or referral for endodontic treatment, to preserve tooth structure and prevent complications like pulpal involvement and dental abscess formation.

**Correct Answer:** D) If the extension of a cavity is more than 2/3, it is considered a deep carious lesion.