Final answer:
The security development principle known as defense in depth emphasizes the use of multiple layers of controls and countermeasures to enhance security.
Step-by-step explanation:
The correct answer is a) Defense in depth. Defense in depth is a security development principle that emphasizes the use of multiple layers of controls and countermeasures to enhance security. By implementing overlapping layers of security measures, any breach or failure at one layer can be mitigated or compensated for by other layers, thereby increasing overall security.