Which of the following helps improve the security of an application when making HTTP calls?

The OWASP Secure Headers Project is focused on implementing secure HTTP headers to enhance web application security. It provides guidelines on which headers to use and how to configure them properly to protect applications from various attacks such as Cross-Site Scripting (XSS), Clickjacking, and other types of injection attacks. By employing these headers, applications can enforce security policies, control access, and mitigate risks, thus greatly improving the overall security posture of any application making HTTP calls.

In contrast, response error codes can provide information about the success or failure of requests but do not actively contribute to improving security. Testing frameworks are essential for validating application functionality and performance rather than directly addressing security measures. Application performance monitoring, while crucial for ensuring the smooth operation of applications, focuses more on tracking and improving application efficiency rather than security enhancements. These distinctions clarify why the OWASP Secure Headers Project is the most relevant choice for enhancing application security in the context of HTTP communications.