Correct option is (d) 20-30 cm
To explain: Cutter bar is an assembly comprising of fingers, knife guides, on wearing plates and shoes. It is used for cutting grasses and forge. It is made of high grade steel. It works like a knife. There are ledger plates provided with the knife guard, on which the knife moves. Knife clips hold the sections down against the ledger plates. Knife clips are placed with wearing plates spaced 20 to 30 cm apart.