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Any substance that suppresses CDKs or known as cyclin-dependent kinases is a CDK inhibitor. They're used to fight cancer because they prevent the cancer cells from multiplying uncontrollably.

One of the most important roles of p53 in mitosis and cell division is that it activates when the cell's DNA has been damaged during G₁'s preparation steps.

It is known that the p21 gene is activated in response to the p53 protein. By binding to and limiting the activity of cyclin/CDK complexes, the p21 protein is an important negative regulator for cell growth and cell cycle progression.

Thus, the correct option is - (d).

Cdks are only able to control the cell cycle if they are securely linked to cyclins. So, cyclins are not CDK inhibitors.

Thus, option (a) is not correct.

The anti kinase is an inhibitory enzyme that inhibits the function of the kinase. So, these are not CDK inhibitors, instead, they are kinase inhibitors.

Thus, option (b) is not correct.

Rb regulates the activity of other proteins that act as positive regulators by keeping tabs on the size of the cell. So, these are not CDK inhibitors.

Thus, option (c) is not correct.