I'm fairly sure that the rocket's acceleration is exactly equal to it's force divided by it's mass - It's orientation is irrelevant.
E.g. If it were an unusually heavy rocket with a relatively weak chemical propellant, such that it only just managed to "hover" when aimed vertically up, then it's acceleration would still be the same 9.8m/s/s as when it appeared to rapidly disappear from sight when aimed horizontally toward the east.
Judging by the answer Stephen gave I would postulate the the question posed should have been: "To an outside observer standing still on the Earth, in which direction would the rocket appear to accelerate the most?"
You could have got another trap out of that one!
