Explanation:

The Abacus is universally acknowledged as the earliest known calculating device, with its origins tracing back to ancient Mesopotamia, evolving significantly in various cultures including China. It was used for performing basic arithmetic operations such as addition, subtraction, multiplication, and division by manipulating beads on rods. Option (a) Clock is a device for measuring and indicating time, not primarily for calculation. Option (b) Difference Engine was a sophisticated mechanical calculator designed by Charles Babbage in the 19th century, centuries after the Abacus. Option (d) Calculator refers to modern electronic or earlier mechanical devices that perform arithmetic, all of which are much more recent inventions than the Abacus. Therefore, the Abacus ($ \text{c} $) stands as the foundational calculating tool.