Answer: a.policeman
Step-by-step explanation:
The cell wall of a cell is like a policemen. Like a policemen secure a city by restricting the entry of undesirable outsiders, the cell wall restricts the entry of unwanted molecules which could harm the cell. Also a policemen keeps an eye on departure of the members of the city so as to keep an record. Likewise, the cell wall filters out unwanted molecules out of the cell. A policemen maintains the boundary of the cell and the cell wall provides a shape and structure to the cell.