ANS:D - five times the diameter of the orifice.

An orifice is called a large orifice if the water head is more than five times the diameter of the orifice. A large orifice is characterized by having a water head significantly greater than the diameter of the orifice itself. This condition allows for the discharge coefficient to remain relatively constant regardless of variations in the head or discharge. Therefore, the correct statement is: five times the diameter of the orifice.