Final answer:
Corks with a large surface area are most likely to pop when a piston in a cylinder filled with fluid is hit with a hammer, due to the transmission of pressure throughout the fluid as per Pascal's principle. Thus, the option a is the correct answer.
Step-by-step explanation:
When a cylinder with a movable piston is filled with a fluid and the piston is struck with a hammer, the corks most likely to pop are the corks with a large surface area. This is due to Pascal's principle, which states that a change in pressure applied to a confined fluid is transmitted undiminished to all portions of the fluid and to the walls of its container. When the piston is struck, it rapidly increases the pressure in the fluid. Corks with a larger surface area experience a larger force because pressure is defined as force per unit area. Since the pressure increase is the same throughout the fluid, larger area corks experience more force, leading them to be more likely to pop than corks with smaller surface areas. The position of the corks (at the top or bottom) is less relevant in this scenario as the pressure is transmitted equally in all directions within the fluid.