Final answer:
Tiffany's utility bill is a variable expense as it changes each month, as opposed to fixed expenses like rent and car loan payments which remain constant.
Step-by-step explanation:
The expense that is considered one of Tiffany's variable expenses according to her spreadsheet is the utility bill.
A variable expense fluctuates in amount from month to month depending on usage or consumption. In the given data, we can see that the utility bill amounts vary each month, with figures like $114.11, $118.98, $124.41, and so on, indicating that these expenses change and therefore are variable.
In contrast, expenses like rent and car loan payment, which remain constant over the months, are fixed expenses.