Final answer:
The correct option is A). Humphry Davy's discovery of a new element, potassium, in 1807 was based on the chemical reactivity and inability to decompose the substance further, which fit the definition of an element at the time. This was supported by the broader scientific understanding of elements and atomic theory developed by Dalton and Avogadro.
Step-by-step explanation:
In 1807, when the English chemist Humphry Davy claimed the discovery of a new element called potassium after passing an electric current through molten potassium hydroxide, he relied on the fact that an element is a substance that cannot be broken down into simpler substances. Before the advent of modern instruments, chemists, like Davy, used the chemical reactivity and inability to decompose a substance further through chemical processes as the primary basis to classify a substance as an element. They grouped elements by their similar chemical behaviors, as seen with groups like the alkali metals (lithium, sodium, potassium) or the halogens (fluorine, chlorine, bromine, iodine), which formed consistent patterns in their reactivity and the types of compounds they formed.
This understanding was bolstered by the discoveries and theoretical work of scientists like John Dalton with his atomic theory, and Amedeo Avogadro's contributions to the concept of the mole and Avogadro's number. Hence, despite Davy not having modern instruments, the consistent chemical properties and lack of further decomposability of the bright, shiny reactive substance he isolated were sufficient for him to claim it as the new element potassium.
This foundation of classifying elements based on their chemical reactivity was part of the larger emergence of chemistry from alchemy and was essential for the systematic development of the periodic table of elements.