What an interesting point; I'd never thought about it. When I saw Apple's multiple choice question, I was a little intrigued. My instinctive answer was "that" as it just sounded much better. Still, as I continued to reflect on the other options, "who" and "whom" seemed to be okay too, based on some "rules" about adjective clauses which I have come across in many books. "Rules" can be so dangerous since they rarely account for all possibilities! Thank God gut-feeling can assist us too in times of doubt!
Anyway, it's great when we can see the logic behind a certain point for which we couldn't previously find good, solid reasons – not only as such confirmation feeds our own intellectual hunger, but because it also makes us, well-meaning, non-native English teachers, better prepared in a classroom situation to cope with the onslaught of skeptical, rule-oriented students who keep pressing for black-and-white explanations and giving us a disdainful look when we can't provide any. Actually I sympathize with rule-starved students; when we're learning something for the first time, we can't rely on instinct alone, and to a certain extent, rules do indeed make the whole learning process easier, so I usually try to explain everything as clearly as I can, which is not always easy, depending on the topic being examined.
Though I love English and I love my job and I truly hope I have been helping at least some of my students, when all is said and done, I cannot vouch for their knowledge, neither for that matter do I feel very confident about my own!, as so many different doubts continue to rear their ugly but endearing heads - I can safely say, though, that the more I teach, the more I learn, and all in all the experience has also been very enjoyable!
SÃ£o Paulo, Brazil