Final answer:
The largest portion of federal grants goes to health and human services, as it is one of the top categories comprising a significant portion of federal spending.
Step-by-step explanation:
The question asks to which category the largest portion of federal grants goes. Among the options provided--education, transportation, health and human services, and defense--the correct answer is health and human services. This is because, as described in the reference information, the major categories of national defense, Social Security, healthcare (which includes health and human services), and interest payments together comprise a significant portion of federal spending. With healthcare being one of the top categories, it is safe to deduce that it receives the largest portion of federal grants compared to the other options listed.