The correct answer is: B. Abacus
The abacus is a calculating tool that was invented in Babylonia over 2,000 years ago. It is still in use today in some parts of the world. The abacus is a digital computer because it uses digits to represent numbers. It is also a mechanical computer because it uses physical objects to perform calculations.
The ENIAC, EDVAC, and EDSAC were all early electronic computers that were developed in the 1940s. They were not digital computers because they used analog signals to represent numbers. Analog signals are continuous waves, while digital signals are discrete pulses.
The abacus is the earliest device that qualifies as a digital computer because it uses digits to represent numbers and it is a mechanical computer.