Between the bounds
If the student was clearly unsure, then:
- Lower bound is whatever they'd get for selecting the incorrect method but otherwise showing correct workings, etc: ideally they should get more than that, unless there's a very strict existing rule for "only first answer counts".
- Upper bound is whatever they'd have got for using only the correct method; they should get less than that.
- Either way, subsequent questions' grades shouldn't be affected by using the incorrect values from this question: any penalty should be applied only once.
S - any score for "showing workings" or other stylistic points. A base value that is given even with an incorrect selection of methodology, so long as they show good workings for some methodology.
C - any score for giving the correct answer. S + C should be the total points for this question.
N - number of guesses.
P - Problem space untouched by their guesses. Assume a value of 1 for now (very large problem space).
The rubric would then be S + P(C/N).
In the degenerate case where there are no style points to be had, P is 1, and they made only two guesses, you'd give 50%. They're not "entirely wrong", even if the wrong answer was the first; but they're not "entirely right" either.
P is, more correctly, the portion of incorrect problem space left untouched by their guesses. For large problem-spaces like "what method should you use for this question?" you can treat it as 1. But in a multiple choice question of A/B/C/D, with A correct, the incorrect problem-space is only three items long, B/C/D. So if they guess "A or B", this would use a third of the incorrect problem-space, giving P=2/3, for a total P(C/N) score of only 33%. And if they guess both possible answers in a true/false question, then P=0, so they get 0%.
It is possible, however, that by writing "it is either X or Y", the student was instead asserting "the question does not give sufficient information to select between these two approaches, both of which could be considered valid depending how you read the question."
If that might be a valid opinion about the question (if the question could be read so that the incorrect case might apply) then you'd most likely need to award full marks, plus extra credit for being the only one to spot that.