To solve this problem, we need to determine at which point in the cell cycle some dividing cells exit and enter the quiescent stage, known as the $G_0$ phase.$\newline$Let's analyze the cell cycle stages:$\newline \bullet$G$_1$ phase:$\newline$ - This is the first gap phase after cell division, where the cell grows and prepares for DNA replication.$\newline$ - Cells can exit the cell cycle from this phase and enter the $G_0$ phase if they are not going to divide.$\newline \bullet$S phase:$\newline$ - This is the synthesis phase where DNA replication occurs.$\newline$ - Cells do not exit to $G_0$ from this phase.$\newline \bullet$G$_2$ phase:$\newline$ - This is the second gap phase where the cell prepares for mitosis.$\newline$ - Cells do not exit to $G_0$ from this phase.$\newline \bullet$M phase:$\newline$ - This is the mitosis phase where cell division occurs.$\newline$ - Cells do not exit to $G_0$ from this phase.$\newline$Therefore, the process of entering the quiescent stage $(G_0)$ occurs at the end of the $G_1$ phase.$\newline$This corresponds to Option 1: $$G$_1$ phase.