The correct option is A Sodium
Electropositivity is the measure of the ability of elements (mainly metals) to donate electrons to form positive ions.
Sodium is present in the first group of the periodic table. It has only one electron in it's outermost orbit hence can easily lose electron to form positive ions. Whereas calcium, aluminium and silicon has 2, 3 and 4 valence electrons respectively. Thus sodium is the more electropositive compared to other options.