Final answer:
The Arduino UNO utilizes the ATmega328p microcontroller, which is favored for its simplicity and the abundance of learning resources available. Therefore, the correct option is A.
Step-by-step explanation:
The microcontroller used in the Arduino UNO is the ATmega328p. This is a popular microcontroller choice for hobbyists and educators due to its simplicity and the vast amount of resources available for learning. The ATmega328p is an 8-bit microcontroller with 32 KB of in-system programmable flash, 1 KB of EEPROM, and 2 KB of SRAM. Its ease of use is complemented by the Arduino integrated development environment (IDE), which simplifies programming the microcontroller for a wide variety of tasks.