Forwarding a student’s question about exam modalities to a professor as a TA?

You cannot answer this question, so you have to ask the professor. You do not want to be responsible for the potential confusion caused by a wrong answer. If your professor already made up their mind, it’s a matter of seconds for them to answer the mail (this is a clear case of this PhD comic). If not, they need to make up their mind anyway some time, and you were right to redirect the question.

Check that the forwarded mail is concise; otherwise summarise it when forwarding. Also make sure that the response goes to you as well (include yourself and the student in the reply to field) as you may get further questions along this line.

This does not really apply to this case, but for more complex or exotic questions you can also include a suggested action and rationale and just have it ratified by the professor, e.g.:

A student asked me whether they should bring a print of the formula sheet or it will be provided. I would guess the latter since it makes it easier for us to control that the sheets have not been tampered with. Can you please confirm or correct this?

Since you don't know the answer, you have two options. You can (1) forward the email to your professor asking the answer, then respond to the student after you've heard back, or (2) hand it off to your professor by copying your professor on your reply. Personally, I'd go with the latter, replying something like this:

[adding Professor Jones]

Professor Jones, I don't know how to answer this. Should students print copies themselves?

This gives both your student and your professor a quick handoff and a clear expectation that your professor should reply to both of you with the answer and then you'll both know.