The correct answer is Economy of mechanism.

Economy of mechanism is the design principle that favors simple and small implementations so they are easier to understand test and secure. A simpler authentication design with a single sign on system or a credential manager reduces the number of components and interfaces that must be protected and verified which lowers the chance of configuration errors and vulnerabilities.

Economy of mechanism also makes auditing and formal analysis more practical because there is less code and fewer interaction paths to review. Centralizing authentication into a well designed minimal mechanism supports consistency in credential handling and reduces the overall attack surface compared with many disparate custom solutions.

Component reuse is incorrect because reuse focuses on leveraging existing modules to avoid duplication rather than insisting on minimal or simple designs. Reused components can still create complexity and integration points that contradict the economy of mechanism goal.

Least common mechanism is incorrect because that principle emphasizes minimizing shared mechanisms between users or processes to avoid unintended interference. It is about separation and reducing common dependencies rather than simplifying the overall design to support single sign on or credential managers.

Open design is incorrect because it states that security should not depend on secrecy of the design and that algorithms and designs can be public. It does not specifically promote minimal implementations or fewer components which is the focus of economy of mechanism.

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