The core issue here is cleartext transmission of sensitive data, and option C directly addresses this:
* Addressing the Problem: The scenario reveals the vulnerability is the lack of encryption during data transmission (the GET response). Ensuring encryption in transit fixes this specific exploit.
* Transport Layer Security: Encryption during transit is typically achieved through protocols like TLS (HTTPS), preventing the interception of sensitive information.

The category of secure software best practices being described is Training. This is because the focus is on educating new developers about organizational security policies and coding practices to mitigate potential threats. Training is a proactive approach to ensure that developers are aware of security concerns and are equipped with the knowledge to address them in their coding practices.
References: The importance of training in secure software best practices is supported by industry resources such as the SAFECode's "Fundamental Practices for Secure Software Development" which emphasizes the need for application security control definition and management1, and the NIST's Secure Software Development Framework (SSDF) which recommends integrating secure development practices throughout the software development lifecycle2. Additional support for this category can be found in resources detailing effective secure development practices345.