Which of the following is still useful for adding numbers?

The correct answer is: C. Abacus

An abacus is a calculating tool that was in use in ancient times and is still in use today. It is a frame with beads strung on wires. The beads are moved up and down on the wires to represent numbers. The abacus is a simple tool, but it is very effective for adding, subtracting, multiplying, and dividing numbers.

EDSAC, ENIAC, and UNIVAC are early electronic computers. They were not designed for adding numbers, but they could be used to do so. However, they are no longer in use today.

Therefore, the only option that is still useful for adding numbers is the abacus.