Final answer:
Bob's iTunes purchases with each song costing $3 represented as a function is a linear function due to the constant rate of change in the remaining balance of the gift card.
Step-by-step explanation:
The type of function that describes the situation where Bob has a $60 gift card for iTunes, and each song he downloads costs $3, is a linear function. In a linear function, the change in the dependent variable (the remaining balance on the gift card) is proportional to the change in the independent variable (the number of songs downloaded). Therefore, if Bob downloads one song for $3, the remaining balance is reduced by $3 each time, indicating a constant rate of change, which is characteristic of a linear function.