Final answer:
Democritus contributed to our understanding of the atom by proposing an early atomic theory, suggesting that all matter is composed of indivisible particles called atomos. This idea is closest to option 'b' formulated atomic theory from the provided options.
Step-by-step explanation:
Democritus, a Greek philosopher from the 5th century BCE, made substantial contributions to our understanding of the atom through his atomic theory. Democritus postulated that if you divide a substance enough times, you would eventually reach indivisible particles which he called atomos, derived from the Greek term meaning 'uncuttable' or 'indivisible'. Although he did not have experimental evidence to support his ideas, Democritus theorized that these atomos recombine in different ways to form all the varieties of matter, and that properties such as color and taste were due to the arrangement of these atoms, not properties of the atoms themselves. This concept laid the groundwork for later scientists, who expanded upon his ideas with empirical data.
Despite Democritus's influential ideas, options 'a' Discovered the electron, 'c' Described the nucleus, and 'd' Identified isotopes, do not pertain to his contributions. Instead, option 'b' formulated atomic theory is the closest to Democritus's role in the development of our understanding of the atom.