The correct answer is C. Francium.
Electronegativity is a measure of the tendency of an atom to attract electrons to itself. The higher the electronegativity, the more strongly an atom attracts electrons. Francium has the lowest electronegativity of any element, followed by cesium and rubidium. These elements are all alkali metals, which have a single valence electron in their outermost shell. This electron is easily lost, leaving behind a positively charged ion. The positive charge of the ion makes it even more attractive to electrons, further reducing its electronegativity.
Fluorine, chlorine, and radium are all halogens, which have seven valence electrons. These elements are very electronegative because they need only one more electron to complete their valence shell. This makes them very reactive and eager to attract electrons from other atoms.
In conclusion, francium has the lowest electronegativity because it has a single valence electron that is easily lost, leaving behind a positively charged ion. This positive charge makes francium even more attractive to electrons, further reducing its electronegativity.