A bank transfer subtracts money from A and adds it to B. Why run both in one transaction?

To make both of the updates run faster on the database server
Both changes happen together or neither does, so no money is lost (Correct option)
Transactions encrypt the data during the transfer of money
To lock the whole bank database while the update is running

So both changes happen together or neither happens; a crash in the middle cannot lose money

A transaction groups several statements into one all-or-nothing unit (atomicity).

If the system crashes after subtracting from A but before adding to B, the database rolls back the whole transaction, so A keeps the money.

You start it with BEGIN, finish with COMMIT, or undo with ROLLBACK.

BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 'A';
UPDATE accounts SET balance = balance + 100 WHERE id = 'B';
COMMIT;

Transaction = all or nothing.