Question

which organ of fish act as rudder

Answer 1

Caudal fin or tail fin act as a rudder of fish.
It is attached to the caudal peduncle which is the narrow part of the body. Dorsal fins and anal fins help to stabilize.
Pectoral and  Pelvic fins act as brakes and help in upward and downward movement.

Answer 2

Caudal fin, which is also known as tail fin, is the main part of fish's body which provides propulsion for the fish's forward thrust. Fish generally use side to side thrust for their locomotion. Most of the fish use undulatory waves that propagate down the body through the caudal fins. This type of undulatory locomotion is known as Body Caudal-Fin swimming.