The restriction point is defined best as the point in the cell cycle a. where cells stop progressing through the cell cycle regardless of the stress they experience b. at which G1 ends and S-phase begins c. after which withdrawal of mitogens will not prevent cell cycle progression d. before which mitogens can not influence cell cycle progression e. after which mitogens can promote cell cycle progression

The G1 phase of the cell cycle refers to the initial phase of the cell cycle or the first phase of the cell division that needs some additional growth element called mitogen in order to carry on the process. However, these mitogens are needed until the cell attains a particular state in the G1 phase.

If the mitogens are withdrawn prior to that state, the cell cycle will get an arrest. Once the cell has reached and passed that state it becomes completely independent of mitogen and does not care for the presence of mitogen. This state is called restriction point, which usually takes place in late G1 phase.

After passing the restriction point, the cell is destined to go through the process of mitosis regardless of the absence of existence of mitogen. Thus, the correct answer is option c.