The formula is:
True alt = Indicated alt + (ISA dev x 4/1000 x ind alt)
Strictly speaking, it's not quite 4/1000, it's about 3.6/1000, but that's close enough. If you you want the full theory, it's in the Oxford Gen Nav notes, which fully cover the principles of slide-rule type nav computers.
However, the whole point of QNH is that your altimeter reads the right value when you touch down at the airport, whatever the actual temperature.
At the airport, they don't know what the sea level pressure is - unless there a convenient mineshaft nearby that goes down there.
So they measure the QFE. But they know that your altimeter won't correct for temperature error. So when they reduce it to sea level (QNH) they use the standard ISA atmosphere, whatever the real temperature.
That way, your uncorrected altimeter will give the airport altitude on touchdown with QNH set.
Nevertheless, your altimeter is still affected by temperature error. But only, now, on the uncorrected bit. That's the bit between you and the touchdown elevation.
In this case, you are (8050-3600) 4450 feet above touchdown.
Now, treat 4450 as the true alt , and work out all the calculations from a datum of 3600, where the altimeter reads correctly.
As I said,
True alt = Indicated alt + (ISA dev x 4/1000 x ind alt)
4450 = Ind Alt + (+15 x 4/1000 x Ind Alt)
4450 = Ind Alt + (0.06 Ind Alt)
4450 = Ind Alt x 1.06
Ind Alt = 4450/1.06
Ind Alt = 4198
Now remember, that's only the difference from 3600. So add 3600 to it.
4198 + 3600 = 7798
Seems pretty close to answer (c) to me.
However, WhiteKnight, I have another point to make. You recently posted something on Polar Stereographic charts. I suspect that the question you posted was wrong. Nevertheless, this forum is not 'wannabes', which caters for JAA questions, but 'Tech Log', which is much broader. Test pilots, flight test engineers, systems designers, and hugely experienced academics and training pilots contribute here - often, as in your case, to queries which are just plain wrong.
You could at least acknowledge their efforts and say "Thank you" on the forum afterwards. You didn't after your last post.
Try and be a bit more gracious and gentlemanly in future. It doesn't cost much to say thank you - especially when it's you who made the work by getting the detail wrong. It's a lesson for life - not just pprune - if you want to make a success of this profession.
And post JAA questions in 'wannabes' in future. Save this forum for the highly technical stuff.