The hyphen or lack of same is of no consequence in A and C. The hyphen is required if confusion ensues without it. I would be more interested in seeing a hyphen in B to avoid the perceived paradox of 'a two'.
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exodejavua. It is a two hour drive from [Place A] to [Place B].C. is the best alternative. A. is fine with me but some may object to the lack of a hyphen. As Mr M has said, the sentence is intelligible without a hyp
b. It is a two hour's drive from [Place A] to [Place B].
c. It is a two-hour drive from [Place A] to [Place B].