I do not know which answer is correct?
(I suppose the first one shows how likely it is that he is doing that activity right now)
A: He does not answer his phone!
B: He must be driving or He must drive
He must be driving is correct in this context. It implies that he is actually driving now and that is the reason he cannot answer.
He must drive has two possible other meanings.
Either it is a necessary activity “He lives right out in the country so he must drive to get anywhere” [but he is not necessarily driving right now]
Or “He must drive everywhere* – He has a compulsion to drive everywhere [but doesn’t have to do so]
There is also drive as used in Golfing “To get off the green he must drive the ball up the fairway.