Finding a cure for cancer will be a huge ____________ breakthrough for mankind.

The correct answer is "medical," which is the adjective form of the noun "medicine." In this context, "medical breakthrough" refers to a significant advancement or innovation in the field of medicine specifically relating to health and treatments. The phrase "medical breakthrough" is widely used to describe important developments, particularly ones that have the potential to significantly impact healthcare or improve treatments for diseases, such as cancer.

The other options do not fit grammatically in this context. "Medicines" is a plural noun referring to substances used to treat illnesses, which doesn’t align with the idea of a breakthrough. "Medically" is an adverb and would not appropriately modify "breakthrough." "Medicine," while a noun that can refer to the field or the practice itself, would not correctly modify "breakthrough" without an additional adjective. Thus, "medical" is the only option that properly describes the nature of the breakthrough being referred to.