Yeah, its the syllabus structure:
you do the 3 core topics:
Space
Motors/Generators
Ideas to Implementation
then you have to do 1 of the option topics....in the hsc exam, the final 25% is for the option topic, so treat it just as seriously as the other topics.
for the option topic, i highly recommend medical physics.
it's easy, shorter than some of the others, and kind of interesting. and despite what others say, i think there are a fair amount of resources available, so no need to worry about that.
so medical physics is the best option - why burden yourself with more work trying to do harder ones when you are only going to get the same amount of marks?