Comprehensive Detailed Explanation:The most effective and economical way to ensure the security of an automated information system is to design it with security in mind from the outset. This is often referred to as " security by design. " Here’s a breakdown of each option and why option A is correct:

A. Originally designed to provide necessary security

Explanation: Systems designed with security from the beginning integrate secure practices and considerations during the development process. This approach mitigates the need for costly and complex retroactive security implementations, which are common in systems where security was an afterthought.

Cost Efficiency: Security implementations at the design stage can be embedded into the system architecture, reducing the costs associated with later modifications.

Effectiveness: Security-by-design approaches often result in robust systems that are more resilient to vulnerabilities because they address security concerns at each development phase.

B. Subjected to intense security testing

While rigorous security testing (such as penetration testing and vulnerability assessments) is essential, it is reactive. Security testing is more effective when applied to systems already designed with foundational security principles, ensuring that tests identify potential flaws in an inherently secure system.

C. Customized to meet specific security threats

Customizing security to meet specific threats addresses unique risks, but such a targeted approach may miss new or emerging threats not initially considered. It also risks neglecting fundamental security practices that apply universally, leading to potential vulnerabilities.

D. Optimized prior to the addition of security

Optimizing a system before adding security features may enhance performance but does not guarantee security. Security cannot be effectively added onto a system as an afterthought without incurring additional costs or creating potential weaknesses.