A daughter gave her debit card to her mother to use for a few transactions. While the card was in the mother's possession, the card was either lost or stolen. Who is liable for the transactions that occurred after the card was lost/stolen?

Mother doesn't have any liability. Daughter, as account owner, has all of her rights under §§ 205.6 and 205.11. Once you learn of the loss or theft of the card, you (the bank) obviously shuts down the card to prevent further use. Once there is a claim that one or more transactions were not authorized, you have to complete an investigation. Counting the transactions made by your customer or her mother as authorized, you then have to determine the customer's liability for the unauthorized transactions under the rules of § 205.6, remembering that the key date for the $50 liability is the date on which your customer learned the card was lost or stolen.