Reducing the success rate of SQL injection attacks is focused on minimizing vulnerabilities within the application's database interactions, rather than the broader server or network services. SQL injection prevention techniques typically involve input validation, parameterized queries, and the use of stored procedures, rather than changes to the network or server configuration.

A) Closing unnecessary application services and ports on the server is a general security best practice to reduce the attack surface but does not directly impact the success rate of SQL injection attacks. This action limits access to potential vulnerabilities across the network and server but doesn't address the specific ways SQL injection exploits input handling within web applications.

B) Automatically locking a user account after a predefined number of invalid login attempts within a predefined interval can help mitigate brute force attacks but has no direct effect on preventing SQL injection, which exploits code vulnerabilities to manipulate database queries.

C) Constraining legitimate characters to exclude special characters and D) Limiting the length of the input field are both direct methods to reduce the risk of SQL injection. They focus on controlling user input, which is the vector through which SQL injection attacks are launched. By restricting special characters that could be used in SQL commands and limiting input lengths, an application can reduce the potential for malicious input to form a part of SQL queries executed by the backend database.