Which of the following is one of the earliest computing devices?

The abacus is recognized as one of the earliest computing devices due to its long history and its function as a physical tool for performing arithmetic calculations. Originating thousands of years ago, the abacus consists of beads strung on rods, and it allows users to manipulate the beads to represent numbers and perform mathematical operations such as addition, subtraction, multiplication, and division.

In contrast, the other options represent more modern technology that evolved long after the abacus. Smartphones and tablets are advanced computing devices that serve multiple functions, including communication and internet access, but they emerged in the 21st century. The personal computer, while revolutionary in its impact on computing and the workplace, was developed in the late 20th century. Therefore, the abacus stands out as the correct answer among these choices due to its historical significance as an ancient computational tool.