Given Cassandra's tendencies and habits, the best method for her to keep track of her financial records would be:
d. Carrying a notebook and immediately recording all of her transactions in it.
This option takes into account her daily fatigue and the likelihood of losing paper. By recording transactions as she makes them, she can avoid the effort of organizing later and ensure she captures every expense without accumulating unfiled receipts. This also allows her to keep all information in one place without relying on technology, which might be overwhelming for her at the end of a tiring day.