Thatīs a peculiar question...
The question seems to be strictly about the function of the outflow valves, not about the effect of their actions. As soon as the PLA exceeds 15°, they will start to work in the described manner, but what this results in depends on whether the bleeds are on or off during T/O. So I would rather lean to answer B here. If the question was about when the cabin will begin to pressurise as a result of this, I would think C might be better.
Now, I am not sure whether there is a supplement for a Bleeds on T/O on the -300 (it is a while ago since I last flew this type); my company did not apply it and bleeds were switched off during the Before T/O check and back on at 400ī AAL.