What information can be searched using an application serial number?

The correct choice highlights that using an application serial number allows for searching proceedings related to the trademark. This includes access to information about any ongoing or past legal challenges, oppositions, or any other formal disputes involving the trademark application. The application serial number serves as a unique identifier for a trademark application, making it easier to track its status, including any relevant proceedings that may impact its registration or use.

It’s important to note that while competitors' details can sometimes be indirectly inferred from trademark applications or their statuses, they are not specifically searchable using an application serial number. Similarly, approval timestamps might be relevant but are not directly linked in the same way that proceedings are. General trademark history is broader and would require additional searches or methods to acquire, rather than being specifically indexed under the application serial number alone.