The correct answer is B. 0.855.
The coefficient of contraction is a dimensionless number that is used to describe the ratio of the area of the jet issuing from a nozzle to the area of the nozzle itself. For an external cylindrical mouthpiece, the coefficient of contraction is 0.855. This means that the area of the jet is 85.5% of the area of the nozzle.
The coefficient of contraction is important because it affects the velocity of the jet. The higher the coefficient of contraction, the higher the velocity of the jet. This is because the higher the coefficient of contraction, the more of the fluid is concentrated in the center of the jet. This results in a higher velocity of the jet.
The coefficient of contraction is also important because it affects the pressure of the jet. The higher the coefficient of contraction, the lower the pressure of the jet. This is because the higher the coefficient of contraction, the more of the fluid is concentrated in the center of the jet. This results in a lower pressure of the jet.