Final answer:
The statement in question refers to the Second Law of Thermodynamics, which explains the inherent limitations in converting heat energy to work.
Step-by-step explanation:
The statement that “heat energy cannot be completely transformed into work” is a statement of the Second Law of Thermodynamics. Specifically, this pertains to the notion that it is impossible for heat transfer from a single reservoir to be entirely converted into work in a cyclical process where the system returns to its initial state. This is a fundamental concept in thermodynamics that explains the limitations of energy conversion in thermal systems, and is represented by several equivalent expressions such as the Kelvin-Planck statement, stating that the conversion of heat from a single source into work without any other effect is impossible.