A power cut interrupts a bank transfer after the debit but before the credit. Which ACID property ensures the debit is undone too?

Atomicity is the all-or-nothing promise. A transaction either commits all its changes or none. The half-completed transfer (debit done, credit pending) violates atomicity, so on restart the DBMS uses its log to roll back the partial debit. Consistency, isolation and durability deal with invariants, concurrency, and crash survival of committed work respectively.