Fastair,
This is a vector resolution question requiring the CRP5.
First you need to grasp Buys Ballots Law fully. It applies equally to thermal winds and geostrophic winds.
In essence with the wind to your back in the Northern hemisphere the low temperature (for a thermal wind) or the low pressure (for a geostrophic wind) will be to your left.
The question states that the low temperature is to the Northwest, so placing that out to your left would mean the wind hitting your back is coming from the Southwest. Hence the thermal wind is 225/20.
However the geostrophic wind and the thermal wind will interact with each other.
Putting the geostrophic wind on the CRP5 (use the high speed side of the slide and mark the wind down from the centre point) as 210/60. You then turn the wheel to the thermal wind direction of 225 degrees. Move the slide so that your wind mark falls on the very top line of the chequerboard in the bottom section of the slide. This will now allow you to put a second wind mark directly below your first and 2 divisions below it corresponding to a 20 kt wind.
This second mark is now your resolved wind and you should find that if you turn the wheel to put it directly beneath the centre point you should now be able to measure of the combined wind of 213/80.
Hence the answer is B) 210/80
I'm pretty confident that this is right as I only sat my met today and it's still pretty fresh in my mind. I don't believe there are any questions on the origins of thermal winds, but a good grasp of Buys Ballots Law for geostrophic and thermal winds is essential for the exam (quite a few questions on it today).
Finally, I found the Oxford Met CD excellent in understanding Met as moving pictures make the subject much easier to recall, and I would highly recommend the Bristol Online Question database as a revision tool.
Hope this helps,
Obs cop