ANS:D - All the above.

1. The statement "It converges up to Venacontracta and then diverges" is correct. In a convergent-divergent mouthpiece, the flow converges to a minimum area known as the vena contracta and then diverges to a larger area downstream. This design allows for efficient conversion of pressure energy to kinetic energy and subsequent recovery of pressure energy.
2. The statement "In this mouthpiece there is no loss of energy due to sudden enlargement" is not accurate. While the convergent-divergent design aims to minimize energy losses, there is still some loss of energy due to sudden enlargement at the diverging section. However, the design aims to minimize these losses compared to other types of enlargements.
3. The statement "The coefficient of discharge is unity" is not always true for a convergent-divergent mouthpiece. The coefficient of discharge depends on various factors including the design, flow conditions, and geometry of the mouthpiece. It may or may not be unity and varies based on the specific circumstances.