Final answer:
When an 'S' appears on the screen of a coin machine, you should seek assistance from a bank employee to avoid potential machine malfunction or deposit issues.
Step-by-step explanation:
If there is an "S" on the screen of the coin machine, the best course of action is to seek assistance from a bank employee (option a). This could indicate a service mode or a need for maintenance. Ignoring it is not advisable because it might result in the machine malfunctioning or not counting your deposit properly. Inserting your coin without resolving the issue could lead to a loss or miscount of your deposit. Shutting down the machine and reporting the issue could be an overreaction if the situation simply requires a quick fix from a trained employee.