Generally, what causes USCIS Lockbox to reject adjustment applications as improperly filed?

The option regarding the application not being properly signed by the applicant is correct because proper signing is a fundamental requirement for many USCIS forms, including those related to adjustment of status. A signature verifies that the applicant has provided accurate information and has reviewed the application. If the signature is missing or not executed correctly, the application cannot be processed, as it indicates that the applicant has not formally endorsed the content of the application.

In the context of USCIS requirements, having placed the application at the right location, submitted it with the required fee (unless a fee waiver is applicable), and ensured that an immigrant visa is available are all essential aspects of the filing process. However, these do not specifically pertain to the procedural correctness of the document itself. Without the required signature, the application is deemed improperly filed and subject to rejection regardless of adherence to these other criteria.