Explanation:

To understand why the possible value of the velocity of the flow at the outlet of a Kaplan turbine is 0.8V, let's first understand the working principle of a Kaplan turbine.

Working Principle of a Kaplan Turbine:
A Kaplan turbine is a type of axial-flow turbine used for generating hydroelectric power. It consists of a runner with adjustable blades that are designed to operate under low heads and high flow rates. The flow enters the turbine tangentially and passes through the runner blades, causing the rotation of the turbine shaft and generating power.

Velocity Triangle:
To analyze the flow through the turbine, we use the concept of the velocity triangle. The velocity triangle is a graphical representation of the velocities at the inlet and outlet of the turbine. It consists of three vectors: the absolute velocity (V), the relative velocity (W), and the axial velocity (U).

Velocity at the Inlet:
At the inlet of the Kaplan turbine, the flow enters the runner tangentially. The absolute velocity (V) at the inlet is the vector sum of the axial velocity (U) and the tangential velocity (Vt). Therefore, V = U + Vt.

Velocity at the Outlet:
At the outlet of the Kaplan turbine, the flow leaves the runner with a different velocity. The absolute velocity (V) at the outlet is the vector sum of the axial velocity (U) and the tangential velocity (Vt'). Therefore, V = U + Vt'.

Explanation of the Correct Answer:
In the given question, it is mentioned that the velocity of the flow at the inlet of the Kaplan turbine is V. Since the flow passes through the turbine and generates power, there will be a decrease in the velocity at the outlet compared to the inlet. This is due to the conversion of kinetic energy into mechanical energy.

Among the given options, the possible value of the velocity of the flow at the outlet of the Kaplan turbine is 0.8V. This means that the velocity at the outlet is 80% of the velocity at the inlet. This decrease in velocity is expected in a turbine as the flow passes through the runner and imparts energy to the turbine.

Hence, the correct answer is option 'B' (0.8V).