You definitely raise an interesting point and the type of reasoning you are making use of is certainly relevant to some MCAT style questions. That said, it’s important that your speculations square with the passage and don’t contradict or supersede information we are given within it. In this case, there’s no indication that we should be thinking of pKa as a relevant factor. Further, we’re told fairly clearly that Y32 binds Mg2+-ATP and -ADP at high affinity. Even if we did consider pH driven possibilities, we wouldn’t want to choose that option over the information that the passage includes.Since the pka of phenol is 10, wouldn't the molecule be protonated most of the time at physiological pH (7.4)? This would lead to it mostly only able to bind ADP as opposed to Mg2+-ATP, making B the correct answer
I hope this helps and good luck!