The command semanage fcontext -a -t httpd_sys_content_t '/data/webapp(/.*)?' is correct because it uses the semanage fcontext command, which is responsible for managing file context types within SELinux policy. The -a flag is used to add a file specification, -t specifies the type, and the file path given indicates that the policy change should apply to the /data/webapp directory and all files within. This command ensures that the custom directory has the correct context for serving web content securely according to SELinux policies.