Why C? And why not the others?

Correct answer: C. Denmark

Denmark's Dannebrog - a white Nordic cross on red - holds the Guinness World Record as the oldest continuously used national flag. Danish legend says it fell from the sky during a battle in Estonia in 1219; that story is folklore, but the flag's documented use reaches back at least to the 1370s, when it appeared in the Gelre Armorial, and the current design was formally adopted in 1625. Austria tempts because its red-white-red bands also have medieval origins, but their use as a national flag was interrupted over the centuries. The United Kingdom's Union Jack reached its final form only in 1801, and Sweden's yellow cross on blue was itself modelled on the Danish original.

Source: Guinness World Records: oldest continuously used national flag; Britannica: flag of Denmark