So on this question, it asks what error condition can be immediately ruled out, when troubleshooting a LAN interface operation in full duplex mode. The answer it gives, collisions, is the best answer of those given, however, wouldn't collisions still be a possibility if the LAN segment exceeds the max cable length? Just a couple weeks ago at work, in fact, we had this issue where an end user was connected to their access switch using an Cat5 cable that was probably about 20 - 30 feet over the max length, and they had collision issues.
As is often the case, it is the "nearest to best" or the "best of the correct" answer.
None of the answer take into account Eric the disgruntled ex-employee who may take his garden snippers and chop through the cables. Neither do we know if he has brought in his collection 10 x 48 port switches from his garage and taken the network over the 500 barrier, thus causing several of those mentioned problems to occur.
The 100m is a theoretical number. Sometimes it can be less, sometimes more, but it is not an exact science.
The idea is that if you operate with no hubs, no bridges and just purely routers and switches in with less than 500 hosts and no cables stretching over 100m, there should effectively be no collisions.
P68-69 of Wendell's book explains this is in more detail - but basically a switch in full duplex mode, when you add in the buffering and that each port has its own bandwidth, theoretically collisions will not occur.