Final answer:
The "Next Generation Bell tests" addressed the "freedom-of-choice" loophole by using random measurement settings based on distant astronomical sources, ensuring that measured particles couldn't influence the setup and supporting the validity of quantum entanglement over hidden variable theories.
Step-by-step explanation:
The loophole that the Next Generation Bell tests closed is known as the "freedom-of-choice" or "free will" loophole. This loophole considered the possibility that the measured particles could influence the behavior of the measurement devices, or that hidden variables could affect the selection of measurement settings, thus predetermining the outcome of the experiment and violating the principle of test subjects' independence from the researchers' interventions.
To address this, the Next Generation Bell tests used randomly determined measurement settings derived from distant astronomical sources, effectively making it impossible for the measured particles to have any influence over these settings, since it would require communication faster than the speed of light, breaching the laws of physics as we understand them. This advancement was significant for the field of quantum mechanics, as it strengthened the validity of quantum entanglement and further discredited hidden variable theories.