Why D? And why not the others?

Correct answer: D. Paraguay

Paraguay's red-white-blue tricolour is unique among national flags: the front carries the national coat of arms - a golden star ringed by a wreath and the words 'República del Paraguay' - while the back carries an entirely different device, the treasury seal, showing a lion guarding a staff topped with a liberty cap above the motto 'Paz y justicia' ('Peace and justice'). The two-faced design was confirmed by Paraguay's first flag law in 1842, and every properly made flag must still be stitched with both emblems. Argentina and Uruguay both display a golden sun, but identically on both sides, and Bolivia's coat of arms likewise appears the same front and back.

Source: Britannica: flag of Paraguay (obverse arms vs. reverse treasury seal, 1842 flag law)