Final answer:
A button serves as an example of an external interrupt for an Arduino, which interrupts the microcontroller to execute specific code upon being pressed.
Step-by-step explanation:
Among the following options, a button is an example of an external interrupt for the Arduino. When a button is pressed, it causes the microcontroller to pause its current tasks and execute a function or a block of code, which is referred to as an Interrupt Service Routine (ISR). An external interrupt is triggered by an external event, like a button press, rather than by events happening inside the microcontroller, such as a timer overflow.