AWS WAF is a web application firewall that helps protect web applications from common web exploits that could affect application availability, compromise security, or consume excessive resources. AWS WAF allows users to create rules that block, allow, or count web requests based on customizable web security rules. One of the types of rules that can be created is an SQL injection rule, which allows users to specify a list of IP addresses or IP address ranges that they want to allow or block. By using AWS WAF to protect the application, the company can prevent SQL injection and other web-based attacks from reaching the application and the database.

RDS parameter groups are collections of parameters that define how a database instance operates. Users can modify the parameters in a parameter group to change the behavior and performance of the database. By using RDS parameter groups to configure the security settings, the company can enforce best practices such as disabling remote root login, requiring SSL connections, and limiting the maximum number of connections.

The other options are not correct because they do not effectively protect the application and the database from SQL injection and other web-based attacks. Using security groups and network ACLs to secure the database and application servers is not sufficient because they only filter traffic at the network layer, not at the application layer. Using AWS Network Firewall to protect the application and the database is not necessary because it is a stateful firewall service that provides network protection for VPCs, not for individual applications or databases. Using different database accounts in the application code for different functions is a good practice, but it does not prevent SQL injection attacks from exploiting vulnerabilities in the application code.