Final answer:
Pin 13 on a standard Arduino board is the one with a built-in LED, which can be controlled for debugging and testing purposes.
Step-by-step explanation:
The pin that has a built-in LED on a standard Arduino board is Pin 13.
Many Arduino boards come with a built-in LED you can control using this pin. When you set Pin 13 to HIGH, the LED turns on, and when you set the pin to LOW, the LED turns off. This is useful for debugging programs without the need for external circuits or components. You can easily test program logic by making the LED blink, for example. The exact code to blink the LED usually involves initializing the pin in the setup function with pinMode(13, OUTPUT) and then toggling the LED in the loop function with digitalWrite(13, HIGH) and digitalWrite(13, LOW) commands.