Final answer:
The correct ICD-10-PCS root operation for 'Reattachment fourth finger' is 'reattachment' (D), used when a completely severed body part is restored to its normal location.
Step-by-step explanation:
The correct root operation for 'Reattachment fourth finger' used in ICD-10-PCS is reattachment. Reattachment is the operation used when a body part that has been completely severed or cut off is put back in its normal anatomical location. This is different from repair, which is used for restoring a body part to its normal structure or function, reposition, which involves movement such as moving the thumb from opposition back to anatomical position, and transfer, which is not applicable in this context. Therefore, the correct answer is D) reattachment.