Which of the following statements about POST requests is true?

• A. Responses to POST requests are cacheable
• B. Two similar POST calls will result in the same resource
• C. POST is used to update existing resources
• D. POST can overwrite a resource

Answer: (C)

The option that states POST is used to update existing resources is accurate in the context of HTTP methods and RESTful APIs. The POST method is primarily intended for creating new resources, but it can also facilitate updates. When you send a POST request to a resource, the server typically understands the intention to either create a new resource or update the existing one, depending on the resource identifier provided and the server's implementation.

POST requests are not idempotent, meaning repeated identical POST requests can lead to different outcomes, such as multiple resource creations or updates, which is distinct from other methods like PUT or PATCH that are designed for modifications. This flexibility enables usage for updates, making it reasonable to say that POST can be utilized to update existing resources in certain situations.

Responses to POST requests are generally not cacheable due to the nature of their use, which might result in changes on the server side. Similarly, while POST might overwrite existing resources depending on how the server handles it, it doesn't explicitly guarantee that behavior like PUT does, which is specifically meant for updates. Therefore, the assertion that POST is used to update existing resources recognizes the method's capabilities within the broader RESTful context.