The correct answer is $\boxed{\text{A}. 0.611}$.
The coefficient of contraction is a dimensionless number that is used to characterize the contraction of a fluid jet issuing from an orifice. It is defined as the ratio of the area of the jet at the vena contracta to the area of the orifice. The theoretical value of the coefficient of contraction for a sharp-edged orifice is 0.611. This means that the area of the jet at the vena contracta is 0.611 times the area of the orifice.
The coefficient of contraction is affected by a number of factors, including the shape of the orifice, the Reynolds number, and the surface roughness of the orifice. The theoretical value of the coefficient of contraction is only valid for a sharp-edged orifice in an inviscid fluid. In practice, the coefficient of contraction is always less than the theoretical value due to the effects of viscosity and surface roughness.
The coefficient of contraction is used in a number of engineering applications, such as the design of orifice meters and nozzles. It is also used in the study of fluid dynamics.