I realize that this is an older post, but I'll respond in the hopes it helps others studying for the exam.
In my opinion this question is very difficult, especially to solve during an exam. However, I think that one of the easiest ways to approach this may be to realize that options d and e are only possible if there is a phase difference between the waves. Thus, it cannot be d or e.
To separate between a, b, and c I think it's first helpful to write out the two equations:
where A is an arbitrary amplitude
meaning that the slope is given by
The importance here is that this slope is certainly not what would be expected for a normal sine or cosine curve. Thus, options b and c are excluded, leaving only a.