**Core Concept**
The relationship of the denture base to the surrounding mucosa is crucial in resisting dislodgement of the denture, particularly in the horizontal direction. This is due to the anatomical and physiological characteristics of the oral mucosa and the forces acting on the denture.

**Why the Correct Answer is Right**
The correct answer is related to the **border seal**, which is the area of contact between the denture base and the oral mucosa. A well-designed border seal helps to prevent the denture from being dislodged horizontally by creating a pressure differential that prevents air from entering the space between the denture and the mucosa. The border seal is formed by the **mucostatic seal**, which is the result of the denture base being in close contact with the mucosa, and the **mucodisplacive seal**, which is the result of the denture base displacing the mucosa to create a seal.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specifically relate to the resistance of the denture base to horizontal dislodgement. While the **occlusal plane** is important for the overall stability of the denture, it is not directly related to the border seal.

**Option B:** This option is incorrect because it does not accurately describe the relationship of the denture base to the oral mucosa. While the **mucosa** is an important factor in the border seal, the term **mucosa** is too general and does not specifically relate to the resistance of the denture base to horizontal dislodgement.

**Option D:** This option is incorrect because it does not accurately describe the relationship of the denture base to the surrounding mucosa. While the **tongue** can influence the stability of the denture, it is not directly related to the border seal.

**Clinical Pearl / High-Yield Fact**
A well-designed border seal is essential for the stability and retention of a denture, particularly in patients with limited oral mucosa or those with a high risk of denture dislodgement.

**Correct Answer: C. Border seal.**