Final answer:
When the tap of a tank filled with water is opened, the water level will decrease as the water flows out until the gauge pressure at the tap reaches zero, illustrating the relationship between fluid pressure and water flow.
Step-by-step explanation:
When the tap of a tank filled with water is opened, the water level in the tank will decrease. This occurs because the water that was initially contained within the tank begins to flow out due to the force of gravity. If we consider an airtight dispenser filled to a certain water level, once the tap is opened, water will flow out until the gauge pressure at the opening of the tap reaches zero. Since the container is airtight and rigid, the pressure at the top of the container will drop as the water level decreases, allowing the flow to continue until the pressure equilibrium is reached with the outside atmosphere. This scenario illustrates the relationship between fluid pressure and water flow in a contained system. In summary, as the water drains out, there will be less water in the tank, which means the water level will indeed decrease (Option 1).