Operational research: difficult in theory;
Mathematical model: comprehensive mathematical knowledge is needed, some are simple, some are difficult, and the exam should be easy to pass;
Finite element method in engineering: difficult;
Function analysis: It needs a solid mathematical foundation, and the exam should not be difficult;
Fundamentals of Modern Mathematics: I don't know what your course includes. It may be a real variable function or modern algebra (if you haven't learned these things before). If it is these two courses or one of them, it will be more difficult.
Since you say that you are not good at math and want to learn it easily, I suggest you choose the first three courses. Of course, it doesn't matter what you say here. It is important to see the substitute teacher. Different teachers in these courses can tell you how difficult it is.
In addition: the finite element course in engineering is not recommended. You haven't even learned the calculation method, and of course you can't understand the finite element method.