The most electropositive element among the following is Cs.
Electropositivity is the ability of an atom to lose electrons and form positive ions. The elements in the periodic table are arranged in order of increasing atomic number, which is the number of protons in the nucleus. Elements with a higher atomic number have a higher electronegativity, which is the ability of an atom to attract electrons.
The elements in Group 1 of the periodic table, the alkali metals, are the most electropositive elements. These elements have one electron in their outermost shell, which they are easily able to lose to form a positive ion. The element with the highest atomic number in Group 1 is cesium (Cs). Therefore, cesium is the most electropositive element.
The other elements in the question are also electropositive, but they are not as electropositive as cesium. Sodium (Na) is in Group 1, but it has a lower atomic number than cesium. Calcium (Ca) is in Group 2, and potassium (K) is in Group 1. These elements have two, three, and four electrons in their outermost shells, respectively. They are less electropositive than cesium because they have more electrons to lose.