Final answer:
Wet floor signage should only be placed out when the floor is wet, such as after mopping or a spill, and not necessarily when it is merely clean. The correct answer to the question is 'when the floor is wet', making option a the accurate choice.
Step-by-step explanation:
Wet floor signage should be placed out to alert and warn individuals of a potential hazard, specifically when the floor is wet. It is a key safety measure to prevent slips and falls. Wet floor signs alert people to be cautious, thereby reducing the risk of accidents on slippery surfaces.
When the floor is wet, for instance, after it has been mopped or if a spill has occurred, is the primary time when wet floor signs must be displayed. This can be during the cleaning process and for a period afterward until the floor has completely dried.
In contrast, the option 'when the floor is clean' does not always necessitate the use of a wet floor sign, unless the cleaning process leaves the floor wet. Thus, the correct answer to the question is option a: when the floor is wet, not when it is merely clean and dry.
Therefore, the appropriate response would be option a, indicating that choice 'c' (both a and b) and 'd' (neither a nor b) are incorrect in this context.