Final answer:
User input validation, secure session management, and enabling logging on the database are the best solutions to address Charles's concern about SQL injection attacks.
Step-by-step explanation:
Charles should implement user input validation to best address his concern about SQL injection attacks. User input validation involves checking and filtering user input to ensure it meets specific criteria or does not include malicious code. This can be done by using input validation libraries or functions, and by employing input sanitization techniques such as parameterized queries or stored procedures.
In addition to user input validation, other solutions that can help address SQL injection attacks include using secure session management to protect sensitive user data and preventing unauthorized access, as well as enabling logging on the database to monitor and track potential SQL injection attempts.