Final answer:
Rubidium (Rb) is the element that is least reactive with other elements.
Step-by-step explanation:
The element that is least reactive with other elements is Rb (rubidium). Rubidium is located at the bottom left of the rectangular section in the two far-left columns of the periodic table, known as the alkali metals. From the given options, Fr (francium) would be the most reactive element.