Assertion A states that when you squeeze one end of a tube to get toothpaste out from the other end, Pascal's principle is observed. This is true because when you apply pressure on one end of the tube, the pressure is transmitted uniformly throughout the enclosed incompressible fluid (the toothpaste in this case) and eventually pushes the toothpaste out of the other end.

Reason R provides the definition of Pascal's principle: "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 principle is directly applicable to the situation described in Assertion A. When you squeeze the tube, you apply pressure on the toothpaste, and this pressure is transmitted uniformly to all parts of the toothpaste, causing it to be pushed out of the other end.

Therefore, Option D is the correct choice as both Assertion A and Reason R are correct statements, and R is the correct explanation of A.