Hi
Out of all the questions in the met unit this was the one I struggled with the most. Not because I don't understand diurnal variation but because there seems to be different ways of working it out depending on what text you have read. I read both Thom and Pratt books which offered different answers based on the same question.
The question came up in my met exam a few weeks ago and in the end it was the only one out of all the questions I had to take an "educated" guess at so I chose the one that had the biggest safety margin and as luck would have it it was the correct answer. I think if you ever get totally stumped in the exam, go for the one that offers the biggest margin of safety.