Basic computing devices: Which of the following remains a useful manual tool for adding numbers even today?

Introduction / Context:
Not all calculation tools are electronic. For centuries, people have used manual devices to perform arithmetic quickly and reliably. This question asks you to identify a tool that is still practically useful for addition drills and teaching number sense.

Given Data / Assumptions:

• We want a device commonly used for arithmetic, especially addition.
• Practicality in education and mental-math training counts.

Concept / Approach:

The abacus is a bead-frame calculator. Operators manipulate beads to represent place values and perform addition, subtraction, and, with techniques, multiplication/division. Many schools still use it to teach arithmetic concepts and speed calculation.

Step-by-Step Solution:

Verification / Alternative check:

Modern 'soroban' (Japanese abacus) competitions demonstrate rapid addition using bead manipulation, proving ongoing usefulness.

Why Other Options Are Wrong:

• ENIAC, UNIVAC, EDSAC: room-sized, early electronic computers; not handheld adding tools.
• None of the above: incorrect because the abacus clearly qualifies.

Common Pitfalls:

• Thinking of historic computers as generic 'adders'—they are impractical for everyday manual addition.

Final Answer:

Abacus.