Pick the easiest module from QM, EM or Relativity.

xRPMx

The situation is messy so I'll avoid details.

I absolutely must do one module this February that is listed as 'level 3' at the Open University without formally taking the prerequisites. I have to teach myself the required mathematics, in two months, to give myself a decent chance of passing. If your only concern is to get the highest mark, which of the following do you choose? Imagine that you're completed your first year, been put in stasis in year 2, and awaken at the start of year 3 knowing none of the year 2 material.

Electromagnetism: http://www3.open.ac.uk/study/undergr...rse/smt359.htm
The Quantum World: http://www3.open.ac.uk/study/undergr...urse/sm358.htm
The Relativistic Universe: http://www3.open.ac.uk/study/undergr...ourse/s383.htm

I'll spare you my opinion since I don't know what I don't know.

mfb

Based on their description, I think you listed them in ascending difficulty.

Electromagnetism does not need much physics as prerequisite. You will need vector calculus, but even there you might get help:
I think quantum mechanics could be more interesting, but it also needs more mathematics, for example vector spaces. It is very useful to know something about Lagrange/Hamilton formalism from classical mechanics as well.

Relativity... it depends on the lecture.
That part can be tricky, as there is a lot of mathematics involved.
Part 2 and 3 are more phenomenological, they should be fine.

Marco9

I've done two of these courses and all of the required year 2 maths. EM is fairly straight forward but you'll need vector calculus,volume and surface integrals and partial differential equations from MST209 and it's pretty heavy going.

QM was easier but maybe thats due to being a little more interesting as well. Good grounding in mechanics from MST209 , bit of algebra from M208 and they teach all the Dirac notation.

Haven't done the Relativistic course but hear it is the hardest of the three and a heavy workload. So if I had to choose between them I'd say QM, if you're just looking at grades and having to bring the maths up to speed.

Good Luck.