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/undergraduate/course/smt359.htm
The Quantum World: http://www3.open.ac.uk/study/undergraduate/course/sm358.htm
The Relativistic Universe: http://www3.open.ac.uk/study/undergraduate/course/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:

The course will provide you with many opportunities to develop your ability in using advanced physics concepts and mathematical techniques (such as vector calculus)

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.

You will learn how a metric is used to describe mathematically a space-time [...] You will then develop an understanding of Einstein’s theory of general relativity as a metric theory of gravity, including the role of the Einstein field equations, some examples of specific solutions

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 that's 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.