**Question:** According to resistance apex locators, the resistance offered by periodontal ligament and oral mucous membrane is:

**Core Concept:**
Resistance apex locators are devices used in endodontics to locate the apical foramen, the small opening at the end of a tooth where the root canal exits. They measure the resistance offered by tissues surrounding the tooth, which is related to their moisture content and density. In this case, we are discussing the resistance offered by periodontal ligament and oral mucous membrane.

**Why the Correct Answer is Right:**
The correct answer is **D**. Resistance apex locators measure the resistance offered by tissues due to their moisture content and density. In the context of periodontal ligament (PDL), it has a high resistance because of its high moisture content and dense collagen fibers. On the other hand, oral mucous membrane (OM) has a low resistance because it is thin, less dense, and has a lower moisture content.

**Why Each Wrong Option is Incorrect:**

A. This answer is incorrect as the resistance offered by oral mucous membrane is low due to its thinness, less density, and low moisture content.

B. This answer is incorrect as the resistance offered by periodontal ligament is high due to its high moisture content and dense collagen fibers.

C. This answer is incorrect as the resistance offered by dental pulp is not relevant to the question, as it pertains to the resistance offered by periodontal ligament and oral mucous membrane.

**Clinical Pearl:**
Apex locators are essential tools in endodontics to guide the dental instrumentation and ensure proper obturation of the root canal. Understanding the resistance offered by different tissues is crucial in utilizing apex locators effectively.