Mental budgeting (A) is best used only with limited resources and expenses.
Mental budgeting is when you organize how you spend or save (and on what) by using only your head, without keeping tracks.
It can be preferable when we are talking about a small amount of money, because:
- the numbers are easier to memorize,
- there are fewer spending/saving categories,
- the consequences, or record-keeping, are less important in the long term.