This word cannot be uttered by a sane man.
If this sentence is changed into the active voice which one is more appropriate and why?
a) A sane man cannot utter this word
b) A sane man will not utter this word
Is there any rule for changing in this type of sentence that the 'modal verb' will be same for both the active and passive voice?
If preserving the meaning of the sentence is your objective for what is more appropriate, then you have to pick the one that changes the meaning of the sentence as little as possible.
In this case, that's a) cannot.
If you cannot do something, then you are incapable of doing it. To say will not simply means that you're not going to—not necessarily because of a lack of ability.
Note, however, that changing from the passive voice to the active (or vice versa) will change the tenor of what's being communicated. If mood is more important than meaning (say you're deciding on a sentence to use when writing a book), then using a different word could be more appropriate. But that's an entirely subjective choice.
This word cannot be uttered by a sane man
has the exact same meaning as
A sane man cannot utter this word.
If "cannot" is the modal that fits the meaning, then "will not" is wrong because "will" is not a synonym for "can." Where did you get the idea that changing voice requires changing a modal?
