Final answer:
The key development in archaeology during the twentieth and twenty-first centuries is the development of radiocarbon dating, as it has enabled precise dating of ancient artifacts and remains.
Step-by-step explanation:
The twentieth and twenty-first centuries have seen many groundbreaking developments in the field of archaeology, but the one most directly attributable to this period is B) The development of radiocarbon dating. This method revolutionized how archaeologists determine the age of artifacts and remains, thereby shedding light on the sequence and dates of historical events. Introduced by Willard Libby in the late 1940s, radiocarbon dating provides a way to date organic materials by measuring the decay of carbon-14. This technique has been indispensable in forging an understanding of historical timelines, correcting assumptions made about the human past and the progress of civilizations.
Other historical landmarks mentioned, such as A) The discovery of the Rosetta Stone, C) The excavation of Pompeii and Herculaneum, and D) The decipherment of the Linear B script, are all pivotal moments in archaeological history; however, they occurred prior to the twentieth century.