That is a good question Viper. I took the Machine Design module even though I mainly design HVAC systems at work. My specialization in school was Machine Design, so I decided to stick with what I received formal education in.
In my line of work we mostly use rules of thumb and basic calculations. After looking at the HVAC material in the MERM, I didn't want to touch that for the in-depth section. During the test, I looked at all three sections for a couple minutes before deciding. Ultimately the Machine Design section seemed easiest to me. For what its worth, I would have rather taken the Thermo/Fluids over the HVAC section after looking at the test.
'.02'
For me, the three sections offered me enough options such that I would have been OK picking any of them, but felt more comfortable with the one that I studied the most.