All cancers begin with a gene mutation giving rise to a faulty protein participating in cell reproduction, even if it is a minor change that occured in this protein. The major problem is that the cell will divide itself nonstop, forming a mass of mutated cells (a tumor). If they start to divide in an undifferentiated way, it can even access other tissues and becom a more serious problem. But, in order to define what causes a tumor at first, the uncontrolled cell division is the beginning of it all.
Therefore, the correct answer is D. uncontrolled cell division