A

In a capillary tube, when a liquid rises due to capillary action, the interaction occurs primarily between the liquid (such as water) and the walls of the tube (which is often glass).
Step 1: The liquid molecules adhere to the surface of the glass due to adhesive forces, while cohesive forces exist among the liquid molecules.
Step 2: The contact surface between the liquid and the glass wall is where these forces play a significant role. This is the primary factor causing the rise of the liquid in the tube.
Step 3: Although there is interaction with air, it does not directly contribute to the capillary rise as much as the interaction between the liquid and the glass does.
Thus, the surface of contact that remains constant primarily involves the glass and the liquid.
Therefore, the correct answer is A.