Final answer:
The most commonly mutated gene in cancer cells is (a) TP53, which encodes the p53 protein crucial for DNA repair and apoptosis in cell cycle regulation.
Step-by-step explanation:
The most commonly mutated gene in cancer cells is (a) TP53. This gene codes for the tumor suppressor protein p53, which plays a critical role in controlling cell division and apoptosis. Mutated TP53 genes have been found in more than half of all human tumor cells, a fact that highlights its significance in cancer progression. p53 is responsible for stopping the cell cycle to allow DNA repair or triggering cell death if the damage is irreparable. In cancers with TP53 mutations, these critical checks are bypassed, allowing cells with DNA damage to continue dividing, leading to the development and spread of cancer.