What is the largest category of federal spending?

Retirement accounts. The most spending is done on social security and federal pension funds.

The federal budget is divided up into three main categories: Mandatory spending, discretionary spending and interest on the national debt. The rules for mandatory spending are already written into law and can only be changed by changing the relevant law. Discretionary spending is the portion of the budget that the president requests and Congress approves or disapproves on a yearly basis. Typically, a little more than 50 percent of the discretionary spending budget goes to the military. Discretionary spending accounts for roughly 30 percent of the total budget. So basically it's called Mandatory Spending