Final answer:
The treatment most effective for managing breakthrough pain in a patient with terminal cancer is supplementing the ATC medication with short-acting opioids.
Step-by-step explanation:
For managing sudden flare-ups of pain known as breakthrough pain in a patient receiving around-the-clock (ATC) medication for terminal cancer, the most effective treatment is supplementing with doses of short-acting opioid. Increasing the dose of her ATC medication may not be suitable since breakthrough pain often requires rapid relief that long-acting medications can't provide. Restricting physical activity could help to some degree, but it won't address the pain directly when it occurs. Doing nothing is not advisable since pain management is a crucial aspect of palliative care, aiming to improve the quality of life for terminal patients.