Safe would mean something like the Treasury Money Market Fund, VUSXX. Unfortunately, it is closed to new investors and the nominal expense ratio exceeds the yield anyway. The default investment is probably Prime Money Market, VMMXX, but the nominal expense ratio again exceeds the yield. There is nothing that is safe, i.e. no risk to principal, other than treasury instruments and the like.
If this is retirement money that's to be invested for a long time, a target date fund will have some risk attached, but woud be an appropriate choice that you can tweak after you decide on your asset allocation.