The correct answer is D. retinal pattern recognition.

In PSP physical security design, access control is part of integrated physical security systems and countermeasure implementation. ASIS describes the PSP credential as covering physical security assessments, integrated physical security systems, and implementation/evaluation of security measures.

Retinal pattern recognition is a biometric access control method. It verifies a person based on a unique physical characteristic rather than something the person carries or knows. Biometric authentication uses unique body characteristics such as retinas, fingerprints, facial structure, or other biological traits for security and authentication.

The other options provide lower security by comparison:

A. Proximity card — possession-based credential; it can be lost, stolen, shared, or cloned.

B. Computer-controlled keypad — knowledge-based credential; PINs can be observed, guessed, shared, or compromised.

C. Smart card — stronger than a basic proximity card, but still a possession-based credential unless combined with PIN or biometric verification.

D. Retinal pattern recognition — biometric; tied directly to the individual and therefore provides the highest level of security among the listed options.

Therefore, for a high-security area such as a data center, the best answer is D. retinal pattern recognition.