Final answer:
When cleaning a patient's dentures, placing a washcloth in the bottom of the sink is the most important step to prevent them from breaking if dropped.
Step-by-step explanation:
The most important step to take when cleaning a patient's dentures is C) Place a washcloth in the bottom of the sink. This practice is crucial because it serves as a safety measure to prevent the dentures from breaking if they are accidentally dropped. A break can not only be costly but also inconvenient for the patient. Therefore, while following proper hygiene procedures, ensuring the protection of the dentures themselves is paramount.