Final answer:
Spinal cord cells remain in G0 phase for most of human adult life due to their post-mitotic nature and lack of regenerative capabilities, which is crucial for maintaining stable neural functions.
Step-by-step explanation:
The cell type in humans that remains in G0 for most of adult life is b. Spinal Cord cells. These cells are post-mitotic, meaning they generally do not re-enter the cell cycle to divide after they have matured.
In contrast to cells like skin or liver cells that can regenerate and thus go through the cell cycle, spinal cord cells do not typically regenerate, making them stay in G0 phase indefinitely. This characteristic likely relates to the need for stability and the lack of regenerative capability in the nervous system, which is critical for preserving neural functions.