The example of a variable expense category from the options provided is birthday gifts.
Variable expenses are costs that can change from month to month, depending on your spending habits, while fixed expenses, like car payments and insurance, remain the same. The gym membership can also be considered fixed if it's a set monthly fee, but it may vary based on usage or membership type in some cases. However, birthday gifts definitely fall under variable expenses since they can fluctuate depending on personal circumstances and preferences.