ANS:B - one-fourth the diameter of the orifice

In a short cylindrical external mouthpiece, the vena contracta occurs at a distance from the outlet of the orifice equal to one-third the diameter of the orifice. The vena contracta is the point in a fluid stream where the cross-sectional area is at a minimum. It occurs just downstream of the orifice or opening through which the fluid is flowing. In the case of a short cylindrical external mouthpiece, the vena contracta forms at a distance from the outlet of the orifice, and this distance is typically one-third of the diameter of the orifice.