Which researcher identified one technical problem that still needs to be solved satisfactorily, but relates to the rotation of the hand when it is placed on the plate?

The correct option is d. Minnaar.

• Minnaar is recognized in the field of forensic science for identifying and discussing practical technical problems related to the capture of biometric data, such as fingerprints and palm prints. The issue of hand rotation when placed on a plate is a common challenge in ensuring consistent and accurate print capture, which Minnaar has highlighted.

The final answer is $\boxed{\text{d. Minnaar}}$.