Final answer:
Quality control or quality assurance is the variable that connects the three entities of the Operations as a Transformation System together.
Step-by-step explanation:
In the context of operations as a transformation system, the variable that connects the three entities together is quality control or quality assurance. Quality control ensures that the operations meet specific quality standards, while quality assurance focuses on preventing defects and errors in the operations. These variables ensure that the machines used in the operations produce goods of the desired quality.