I'm gonna go ahead and say A
I'm have to rule out B because, for me, a platformer is about perfecting one's path through a given level. And if one is OHKO'ed then obviously your path is flawed.
For "unavoidable" and "unreachable" do you mean all and totally unattainable (like a 1up in the middle of the sky with on way to get to it, or a three screen long lava pit with no platform or anything above it), or do you mean something that can be performed if enough practice is applied?
If it's the latter, I can rule out C and D using the same reasoning as B, leaving me with A.
If it's the former, for C and D, I still can't get to/past them therefore I can't go that way and stop trying to, leaving both of them non-issues.
(this is assuming that it is possible to beat a given game)