Oh, I like this poll. It's fun to talk about this stuff.

I had to say "Other reason" because I think it's probably a combination of factors. I doubt a or b are true due simply to universal scale. I figure there is probably extraterrestrial life and furthermore there is probably intelligent life that would be suitably capable of interstellar travel. It would seem statistically unlikely that humans are the "only" or even "the first".

The next question would be do they know fundamental things we don't about the universe and have they been able to exploit that knowledge to develop technology that allows them to travel (safely) between star systems faster than light? If not that would be the "Interstellar travel is too difficult" option, with the difficulty increased if there are no relatively nearby star systems with this hypothetical intelligent life. That seems plausible based on the current human understanding of the universe, but humans have made drastic advances before that largely void the current state of that understanding of "what we think we know now". Even if faster than light travel between systems is possible, the scale of the universe combined with a low (if it is really low) likelihood of life (let alone intelligent life) developing could still make it extremely unlikely for any two intelligent species to find each other. There's also the 4th dimension to consider with time. Perhaps there was or will be life here or there, but not right now while we are looking for them or when they were/will be looking for us, that could be a big killer to different intelligent species ever finding each other as well.

I think "We're too uncool / not worth the trouble" is plausible because any species advanced enough to come to us may not be sufficiently interested by us, but that already assumes there is another intelligent species aware of us and that just seems unlikely to me (maybe I'm too close minded on that). I look at "What do you mean they haven't visited?" in the same way, not entirely implausible (heck sufficiently advanced technology could have them observing us from their own home), but it just seems really unlikely to me.

https://www.youtube.com/watch?v=A4XT8R5AfD8