All of these are hard.
Not sure what yolo YC is but if it's what I think you mean, (corner cutter jumping over the water near the start to avoid going left around the corner and wasting time for quicker access to the tunnel), if done without a Mushroom, that and DDJ bridge option, (if no Mushroom is used to leap the gap on the broken and disconnected bridge), which come to think of it, I think you are referring to the other bridge in DDJ??
Either way, for me doing the two mentioned corner cutters without shrooming are impossible at my level.
Although everything else challenges me, I have and can sub 9seconds on BP flap and subjectively do everything else to a high standard to a degree. BP sub 9seconds flap for me personally, I rate the difficulty about 9/10 which is a bit of a guess as I haven't consistently played that track in a long time. On most maps on MKDD!! Time Trial I feel like I am getting worse due to lack of mental competitive energy and motivation. I have my same skill level but lack energy to motivate myself. Energy improves PRs heaps!!!
While some competitors on the MKPPs may disagree with me about my skill that's how I evaluate my playing strength. I see myself as average among the best and I am fairly happy with it eventhough I really want to be a karting God but that's not the way I was born. It's still nice being world class though

WS glitch is fairly difficult for me eventhough it is probably seen as easy at high level.
DKM gap/corner cutter would be the easist for me out of all these options or dodging traffic which is a bit vague for me. One of those options would be easiest for me but still challenging!! MKDD!! is tough competitively as an understatement.
Perhaps I should have said Rainbow Road is my easiest option but still tough as nails!! Everything you said in the options is hard even the easiest stuff is beyond extremely tough to do well and how well is subjective, very subjective.
For example if a less skilled racer takes a minute to do the DKM gap but doesn't fall off or even far less time in some cases they may have just gone without it. Because of huge time loss. Sure, you can say they did it without falling in the gap but the execution is so poor it's irrelevant they pulled off the DKM gap as EXTREME time was lost. My point is, the execution and the act of the vote both have to be factored into the equation to work out what the hardest vote is to do.
I am average in the competition for AF here, so I can't evaluate the poll properly only what is easiest to hardest for me.
Earlier I said Rainbow Road should be my pick for easiest eventhough it's still very hard!! This is because it's my easiest track to get better results by a huge margin. I don't understand, I am a track specialist at Rainbow Road. Rather I do understand, it's in my genetics somehow to do better at that map in comparison to all other MKDD!! maps and possibly and I mean possibly, (not clear), do better at all other fast times in scores in videogaming.
Not sure as I've achieved a lot in gaming, but I would think it's got a good shot at being my strongest achievement at videogames Rainbow 3laps Time Trial?? Across all videogames I know?? Too hard for me to tell. My point is, Rainbow is hard but it might be the easiest thing for me to do subjectively to a unspecified degree to do it well out of all the votes in the question.
It took a lot of energy for me to write this. For now too lazy to go back over it and check it makes sufficient sense.
Thanks for this fun poll vote

I noticed you are new to the site, welcome and goodluck improving your PRs