I haven't seen you_____ a week.

The given sentence conveys that the subject has not seen 'him' for a length of time. 

Therefore 'for' is the suitable answer, since 'for' is generally used to depict a length of time. Thus option C is correct. 

On the other hand 'within' is used to depict a location in the larger territory. Hence option A is not correct. 

The preposition 'since' is used as a expression of time which mean 'because' and 'from' is used to the starting point of something. Since the sentence speaks about a length of time, 'since' and 'from' are unsuitable in the given context. Hence they are incorrect. 

Thus the completed sentence becomes: I haven't seen you for a week.