I'm under the impression that while you can't bring in new information to a MBT/MSS, the rules are relaxed if the answer choice is a conditional. If the conditional works with the information, the answer choice is correct.
I read B as a conditional. "If there exist people who are very good at manipulating symbols, they do not necessarily have to have any mathematical understanding." I don't see anything in the stimulus to disprove this.
Is the error here that B is not a conditional? Are there conditionals that don't have to use words like "if" or "when"? This is really important because I don't want to get into the habit making a wrong answer choice compatible with the stimulus by turning it into a conditional.
Would D be correct if it read, "Acquiring the ability to manipulate symbols is part
of the process of learning geometry."? This makes it read more like you can't understand geometry if you don't understand manipulating symbols. This would be unsupported and therefore correct.
Learn geometry
Learn to manipulate symbols