Electronegativity is the capability of an element to attract electrons. In this case, the element that gives up its valence electron most readily is the one with the lowest electronegativity. The elements with the lowest electronegativity are the ones that are located further to the right and below in the periodic table.
In this case, Fr is the one that is located the most to the right and below, it means that it will give up its valence electron most readily.
The correct answer is Fr.