A

When one of the holes a, b, or c is unstopped, the water will flow out from that hole due to the difference in pressure and gravitational pull.
Step 1: Understand that a Mariotte's vessel maintains a constant water level as long as the outlet holes are closed.
Step 2: When hole 'a' is unstopped, water will flow out until the water level drops to the level of hole 'a'.
Step 3: For hole 'b', since the water level in tube 'd' is at the bottom, unstopping this hole also allows water to flow out until it matches the level of hole 'b'.
Step 4: Similarly, if hole 'c' is unstopped, water will flow out until the level equilibrates with hole 'c'.
Conclusion: The pressure difference caused by the height of water above the holes drives the flow, which illustrates fluid dynamics principles.