Servitude is the condition of being a slave or of being bound to serve another, with no liberty to act for oneself. An antonym must therefore name the opposite condition, that of being at liberty and subject to no master.

The word that names precisely that condition is the one chosen here, and the pair is a standard opposition in English, since the release of a person from bondage is described as the grant of this very state.

Magnitude refers to size or extent and belongs to measurement, not to liberty. Magnanimity means generosity of spirit, especially towards a defeated rival, and munificence means lavish giving of money or gifts. Both of these name virtues of character; neither is the opposite of bondage, and they are placed here because their shape and length resemble the given word.

Freedom option is correct.