1. Identify the principle involved

   Pascal’s principle states:

   Any change in pressure applied to an enclosed incompressible fluid is transmitted undiminished throughout the fluid and to the walls of the container.

   Mathematically, if pressure change $\Delta P$ is applied at one point in a confined fluid, then the same $\Delta P$ is transmitted everywhere: $\Delta P_{\text{everywhere}} = \Delta P_{\text{applied}}$

2. Check Assertion A

   Assertion A says:

   When you squeeze one end of a tube to get toothpaste out from the other end, Pascal's principle is observed.

   This is essentially correct in the school-level sense. When the tube is squeezed, pressure is applied to the toothpaste inside the tube, and that pressure is transmitted through the paste, forcing it out through the opening.

   So, Assertion A is correct.

3. Check Reason R

   Reason R says:

   A change in the pressure applied to an enclosed incompressible fluid is transmitted undiminished to every portion of the fluid and to the walls of its container.

   This is the correct statement of Pascal’s law/principle.

   So, Reason R is correct.

4. See whether R explains A

   In the toothpaste tube, when one end is squeezed, the applied pressure is transmitted through the toothpaste inside the tube, and this causes toothpaste to come out from the open end.

   Therefore, R correctly explains A.

5. Choose the correct option

   The intended option is:

   • Both A and R are correct, and R is the correct explanation of A.

   From the listed options, this corresponds to Option D (clearly intended despite the typo "Both A and B").

Final Answer

$\boxed{\text{D}}$