His name in Korean is 전정국, if you translate it should become Jeon Jeongguk, but often names are translated in a different way, and often "eo" is written "o" or "u", and the letter ㄱ can ne translated ad a "g" or as a "k", I honestly don't know how people choose which is better, but in this case I'd say it's Jeon Jungkook, his name is usually translated like this, when they debuted there was a lot of confusion and the other options you listed aren't actually wrong and when there are cases like this it's better to wait and see what's the most used translation.
ps. I'm Italian and my English kinda , I'm sorry :\

jungkook is his stage name but jeon jeongguk is his birth name according to wikipidea
u -> eo and oo -> u are just the same tho just with different romanizations
g and k are pronounced the same in korean too i think

jeon jungkook

he himself has chosen this spelling

I mean answer D, not the letter if it's unclear :P

Everything depends on translation you use. Having in mind that not everybody is from english speaking country, so pronounce it differently, I'd say Jeon Jeon Guk is the best optiom. (for example - there is a huge difference in pronaunciation of D for polish person like me - If I'd like to say it as supposed to sound in polish, it'll sound like "", which is totally different than orginal word, doesn't it?)

They're all the same anyway :)

전정국 'eo' and 'u' are the same. so are 'g' and 'k'. but our maknae's name is often written as jeon jungkook.

all of them are pronounced the same way but yeah~ D :3

The company spells it like Jung Kook but it's spelled like Jungguk to fans
Kinda like how Woolim spells Sunggyu like Sungkyu.