**Core Concept:** Posterior teeth refer to the molars and premolars, which are located in the posterior region of the dental arch. The length of the post, also known as the root canal post, is crucial for their proper restoration and long-term oral health.

**Why the Correct Answer is Right:** The correct answer is D: 14-15mm. This length is appropriate for a root canal post because it ensures adequate retention and resistance form, allowing for proper restoration of the tooth's function and preventing further tooth decay or fracture.

The root canal post should extend beyond the cementoenamel junction (CEJ) by approximately 2-3mm to provide resistance form, which is essential for the restoration's stability and strength. Additionally, extending the post beyond the furcation area of the molar tooth prevents fracture due to the excessive occlusal forces.

**Why Each Wrong Option is Incorrect:**

A. 10mm: This length is too short and may not provide adequate retention and resistance form, leading to reduced restoration stability and increased risk of fracture.

B. 20mm: This length is too long and may extend into the periodontal ligament, causing discomfort, inflammation, or even tooth resorption.

C. 12mm: Although longer than option A, this length may not be sufficient to provide the necessary resistance form and retention, compromising the restoration's stability.

**Clinical Pearl:** When selecting a root canal post's length, consider the tooth's anatomy, occlusal forces, and the importance of providing retention and resistance form for optimal restoration success.

**Correct Answer:** D: 14-15mm. The correct length ensures adequate retention, resistance form, and protection of the tooth and surrounding structures, leading to successful restoration and long-term oral health.