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Positive regulators and negative regulators are two families of intracellular regulatory molecules that control the cell cycle. Negative regulatory molecules stall the cell's progress towards the next phase because they prevent it from moving forward.

Retinoblastoma protein, p21 protein, and p53 protein are the most well-studied negative regulatory molecules. Cells that had started to reproduce uncontrolled, i.e. had become cancerous, had all 3 of these regulating proteins damaged or non-functional. Unchecked cell cycle progression was caused by defective copies of regulatory proteins in each instance.

Thus, the correct option is - (c).

The reproductive cells of an organism are known as gametes. Positive regulatory proteins control the creation of gametes and the process of their development instead of a negative regulatory protein.

Thus, option (a) is incorrect.

No processes take place during G0, which happens when cells end the cell cycle. Thus negative regulatory protein does not present in cells in the G0 stage.

Thus, option (b) is incorrect.

Stem cells are one type of cell in the body that have the capacity to differentiate into a variety of different cell types. They act as a kind of bodily healing mechanism. So, negative regulatory proteins are not regulated then.

Thus, option (d) is incorrect.