Imaginative writing and creative writing are essentially the same type of writing. Perhaps you meant to refer to another type of writing?
However, to answer your question, there are a number of advantages and disadvantages associated with choosing imaginative writing.
Advantages:
Imaginative writing, given its nature, offers you more freedom to express your ideas in a creative manner when compared to other forms of writing that might be more restrictive, such as informative writing. This also means that, due to its informal nature, you may have the opportunity to incorporate some form of informal register, which is particularly relevant to creative pieces that feature characters and/or dialogue.
Expanding on the notion of freedom, there is the ability to utilise your own life experiences and combine those with your imagination, allowing you to develop responses that feature sophisticated, ficitional settings (where appropriate).
Lastly, there is an abundance of resources and relevant material targeting creative writing, which can act as a source of inspiration when you are seeking potential ideas or elements that will assist you in developing your own ideas, not to mention that such resources provide you with the opportunity to support the quality of your writing as you closely examine examples of imaginative writing, allowing you to incorporate techniques more thoroughly and effectively for instance.
Disadvantages:
The nature of imaginative writing implies that there is a relatively high level of subjectivity in terms of both developing written responses and marking those responses. Based on this, there is a chance that a piece that you perceive to be of good quality may not be considered of good quality by the person that marks it, possibly leading to inconsistent marking. To mitigate this risk, it is important to ensure that you discuss your ideas and the overall quality of your writing with your teacher and seek advice/suggestions regarding those.
Additionally, because imaginative writing enables you to think freely, there are potentially countless ideas that you could think of. Consequently, this could make the process of developing a good idea rather time-consuming, especially when compared to other forms of writing, where the subject of your response may be specified.
Creative writing appears to lack a proper/standard structure, as opposed to other types of writing, possibly leading to uncertainty in terms of the specific structure/format that you will choose to adopt in your Module C response. If you can effectively manage uncertainty, then this should not be an issue. Otherwise, this may pose an obstacle with respect to imaginative writing.
I hope this helps!