You're right that there is a subtle difference between
"which of the following P's, if valid, justifies the argument"
and
"which of the following P's, if valid, most supports the argument"
In terms of what the question stems literally say, it's akin to the difference between Sufficient Assumption and Strengthen. But Sufficient Assumption and Strengthen questions, while possessing some overlap, are very different question types to me.
Meanwhile, these two different wordings of Principle questions are not really different question types to me.
This is akin to the difference between Inference questions that ask for "what must be true" vs. "what is most strongly supported"
Principle-Justify vs. Principle-Support are the same question type to me, just as Inference-Must be True vs. Inference-Most Strongly Support are the same question.
What we read for and expect is the same for either wording of the same question type, but the correct answer to "most strongly supports" is allowed to be a somewhat looser fit than the correct answer to "justifies"/"must be true".
There are a couple examples of Principle-Support correct answers that just strengthen, without proving the conclusion is true. But the majority of Principle-Justify/Support questions are interchangeable.
I think for this question, (B) still does a good job of locking in what the question stem asks for.
I agree that if we are trying to lock in the logic of Marianne's argument, then we need a connection between 'involuntary' and 'not held responsible'.
But the question stem actually says to support "her argument
against the order".
The order was that she should stop humming or be disqualified, i.e.
Keep humming --> disqualified.
Her argument against the order could be construed as
Humming --> involuntary --> ~held responsible --> ~disqualified
Implicitly, she was arguing that since she's not responsible for her humming, she shouldn't be disqualified on the basis of it.
So a principle that said "An action that someone is not responsible for should not be grounds for disqualification from a chess tournament" would also work here.
Anyway, this is a long-winded response to a question you probably handled with relative ease, so let's not overly dissect it.
Ultimately, on pretty much any Principle Justify/Support question, we just want something that provides connective tissue between the premise idea and the conclusion idea. (B) definitely does this better than the other choices, so we'll take it.
Hope this helps.