Cron,
There could be several questions. For example:
You are flying at 12,000' on a QNH of 1030hPa. If you set your altimeter to to 1013hPa, what altitude would it indicate? The answer is then as per Alexban's reply.
More likely would be to turn it the other way around. You are flying at 12,000' on the standard pressure setting. At what altitude are you flying if the QNH is 1030hPa? This time, the answer is as per Alexban's reply, but add the 510' instead of subtracting it, so the answer is 12510', which isn't even close to any of your options.
Or you could turn it into a temperature correction question by saying that you are at 12,000' on the QNH. The tempareture is ISA-10 degrees. (Temperature correction questions always seem to be colder than ISA, because this is the dangerous one!) The QNH of the local airport is 1030mb (this is a red herring), the altitude of the local airport is 5000' (this isn't a red herring!). What is your true altitude. For this question you need to work out your height above the datum. The datum is 5000', your height above this is 7000'. The error is 4% of this (4% for every 10 degrees), i.e. 280', so the answer is (12,000'-280'=) 11,720'.
Note, though, that in every case I'm telling what your altimeter setting is - the crucial thing which is missing from your question. If you want to add anything about MSA, then go ahead, but that would be a complete red herring - the altimeter doesn't care about any nearby mountains or masts.
I'm sure some of the current groundschool students or instructors will have plenty of "real" feedback questions along these lines.
FFF
--------------