Every program is compiled as though it owns memory starting at address zero, and thousands run at once without colliding. What makes that possible?

C. Virtual memory, each process getting its own translated address space

The MMU translates each process's virtual addresses to distinct physical pages, so identical addresses mean different memory, which gives isolation for free and lets programs be written without knowing what else is running. A wild pointer then hits your own address space rather than someone else's, which is what makes a crash a crash and not a corruption.