The quote that best supports the inference that Henry’s parents are determined to make their restaurant successful is:
“His parents practically lived there; they couldn’t loosen their grip even though they had hired a manager after many stressful days.” (paragraph 1)
This quote indicates the extent to which Henry's parents are committed to the restaurant, as it suggests that they spend nearly all their time there and are deeply involved in its operations, illustrating their determination to make it successful.