Flour. Maybe rice. Something of that nature. The only alternative option that’s even close to food might be fuel, depending on the nature of the disaster.

Water is everywhere outside of deserts, and there’s enough raw materials for a thinned population to recycle for a long time. Food and medicine is what separates the living from the dead, and medicine isn’t a single item. Edit: Although antibiotics or water treatment tablets are probably the closest approximation.

Pleasantly surprised to not see more throwing star-type answers.