Open channel flow refers to the flow of liquid in a conduit that has a free surface. Unlike closed conduit flow, where the liquid is completely enclosed within a pipe or conduit, open channel flow involves the flow of liquid in a channel with an exposed surface. The correct answer to the given question is option 'B' - "The liquid flowing in a closed conduit has a free surface."

Explanation:

1. Closed conduit flow:
Closed conduit flow refers to the flow of liquid in a completely enclosed pipe or conduit. In this type of flow, the liquid is confined within the boundaries of the conduit and does not have any exposed surface. The flow can be pressurized or unpressurized, depending on the conditions.

2. Open channel flow:
Open channel flow, on the other hand, occurs when the liquid flows in a channel or a conduit that has an open or exposed surface. This type of flow is commonly observed in natural channels such as rivers, streams, canals, and man-made channels such as flumes and gutters. The surface of the liquid is exposed to the atmosphere, and the flow is influenced by the gravitational forces acting on the liquid.

Characteristics of open channel flow:
- The liquid in open channel flow has a free surface, which means that it is not confined within the boundaries of a pipe or conduit.
- The flow is influenced by gravity, as the liquid moves under the effect of the gravitational force.
- The velocity distribution in open channel flow is non-uniform, with the highest velocities occurring near the free surface and the lowest velocities near the channel bed.
- The flow is usually open to the atmosphere, allowing air to mix with the liquid.
- The flow can be subcritical, critical, or supercritical, depending on the flow velocity and the characteristics of the channel.

Conclusion:
In conclusion, open channel flow is the type of flow where the liquid flows in a conduit that has a free surface. This is in contrast to closed conduit flow, where the liquid is completely enclosed within a pipe or conduit. The correct answer to the given question is option 'B' - "The liquid flowing in a closed conduit has a free surface."