To solve this problem, we need to understand the characteristics of the $G_0$ phase in the cell cycle.$\newline \bullet$The $G_0$ phase is a resting phase where the cell is not actively preparing to divide.$\newline \bullet$Cells in the $G_0$ phase have exited the cell cycle and are in a state of quiescence.$\newline \bullet$This phase is often considered a point where cells can either re-enter the cell cycle or remain in a non-dividing state.$\newline \bullet$Examples of cells that enter the $G_0$ phase include nerve cells and muscle cells, which do not frequently divide.$\newline$Based on this understanding, the correct option is:$\newline$Option 1: Exit the cell cycle.