Final answer:
Single mothers are the poorest ranked category of single parents due to their relatively high poverty rates, compared to single fathers, grandparents, and foster parents.
Step-by-step explanation:
When considering the poorest ranked category of single parents, data indicates that single mothers tend to have higher poverty rates compared to other groups such as single fathers, grandparents, or foster parents. According to the National Journal and the U.S. Census Bureau, of all U.S. families, a significant percentage is headed by single mothers, and this demographic experiences relatively high poverty rates. In contrast, information on single fathers, grandparents, and foster parents does not suggest that they are the poorest ranked category of single parents. The poverty rate among single-parent households tends to be relatively high, and this is especially true for single mothers, who face various financial challenges.