Index-as-Key for a List Without Unique IDs

It is always acceptable since React re-derives correct identity from element type regardless of key value

It is acceptable only if the list is static and will never be reordered, have items inserted or removed - for any mutable list, a stable key should instead be generated at the point of data creation and stored with the item

It is acceptable as long as the list is short, since reconciliation cost scales with list length rather than key stability

It is never acceptable, and a key must be omitted entirely to force React to always re-render every item