Which of the following expansion slots is 64-bit?

PCI-X is indeed a 64-bit parallel bus, but it is an older, largely superseded standard not typically found on modern motherboards.

Standard PCI slots were available in 32-bit and 64-bit versions, but it is an older, slower standard compared to PCIe.

ISA (Industry Standard Architecture) is an obsolete 8-bit or 16-bit bus, incapable of supporting modern 64-bit computing needs.

PCIe (PCI Express) is the contemporary high-speed serial expansion bus that replaced older PCI and PCI-X standards, designed to support the extensive data transfer requirements of modern 64-bit processors and operating systems. Although architecturally a serial bus, its multi-lane configurations (e.g., x16) provide significantly higher effective bandwidth than older parallel 64-bit buses.