Right, (a) is the correct order. However, it still seems faintly awkward to me, albeit I do not see any other simple way of saying it. That rule is not without exceptions, by the way.
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MIGDuring the chasing, how many hours were Felix and Adam behind Alex?Could we say this?"During the chasing" does not feel terribly natural. You could say "During the chase" or "As/While they were chasing him".