Concept:
A mower is used for cutting forage crops and grasses. Its cutter bar has different parts such as knife, ledger plate, guards, shoes and grass board.
Step 1: The cutter bar cuts the standing crop.
Step 2: After cutting, the crop must be moved aside so that the machine can continue working smoothly.
Step 3: The grass board is fixed at the outer end of the cutting bar.
Step 4: Its function is to shove or push the already cut plants aside.
Step 5: Therefore, the correct answer is:
\[
\boxed{\text{Grass board}}
\]