Misunderstood exam question with consequences for incorrect answer
At my institution the default way to handle this is (3 Stick with the 0 for this question). Essentially, every student is required to handle the teaching language well enough to be able to work on the test. In practice, many teachers will fall back to (2 Give them some consolation points) if giving 0 points seems too harsh. If (1 Give them another stab at it with more language context) is an actual option, that seems like a senseful way, too (e.g., in a short verbal exam). However, in practice this is often not possible in my courses, either because it would be very impractical or because the course regulations do not allow it.

Both, (4 Grade the question they answered and not the question asked) and (5 Give him / her the lowest grade anyone else got for this question), seem like relatively weird ways to handle this situation. With (5), you are essentially decoupling the grades for the student from what (s)he has actually written on the test. (4) breaks a fundamental exam concept, i.e., that the instructor chooses the question that the student should be answering, and not vice versa.

As for this:


  The student is on an important grade boundary and it could make a difference to their final grade even a difference to if they have to take the class again / can graduate on time / or even stay in the major.


As bad as you may feel if "your" grade is the tip of the iceberg that leads to bad consequences for the student, you should be aware that it is the sum of bad performances that has gotten the student into troubles. Your grade is just the last in a series, and your grade is as much "at fault" as any other bad grade the student received. Hence, I feel you are not required to take the larger picture into account.