Final answer:
A significant decrease, but still unacceptably high levels in Africa and Asia.
Step-by-step explanation:
The phrase that best describes twenty-first-century global trends in undernutrition and stunting is a significant decrease, but still unacceptably high levels in Africa and Asia.
According to data from the World Health Organization, there are still millions of people affected by undernutrition and stunting, particularly in Africa and Asia. While there has been progress in reducing undernutrition in some parts of the world, the problem persists and remains a significant concern.