If the goal of the question was to see whether the student is able to choose the correct approach among of few, and from their solutions it is not obvious (to them) which one must have been the correct one, it is not as clear and I would understand deduction of points. However, keep in mind that you generally don't want to punish students for writing down more - you want them to demonstrate what they know, after all. And in my opinion having one correct answer and one wrong answer is still better than having only a wrong answer.
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