Final answer:
The number you type into an ATM to indicate the amount of money you want to withdraw represents an input into the machine's system, making the correct option a. input.
Step-by-step explanation:
When you use an ATM and type the number to indicate the amount of money you want to receive, the device you are using is an input device. An ATM keypad, where the numbers are typed, serves as an input interface for you to communicate instructions to the machine, which it then processes to give you the desired output, in this case, the money you requested. As per the choices provided, the correct answer to what type of device the number you type represents is a. input.