I assume the so-called "my answers" is just your hypothesis, there weren't the alt options, were there?
Yes, that, whom, and NWAB are all perfectly fit in the brackets, too.
The only reason why "who" is chosen to be right answer (as the original sentences put, not actually choice questions, I guess) is, langauges develop. Because everyone accepts it, it's then accepted. As a matter of fact, most natives now even can't be very sure of the use of the word "whom", themselves, including the well educated. Just do a poll or something here.
Funny enough, I thought according to the strictest Chinese-based English grammar rules, the right answer for the first sentence would then def be --- "that", because of the "one of the best" structure thing. Of course, you probably wouldn't've heard of it, as the majority of human beings on earth have not.