Answer:
I would say #2 for purpose alone.
However, you "could" say #4 in the sense that writing can be used to convey shock (but the purpose of writing a simple sentence is not necessarily to convey shock), and while ALL writing should reflect clarity, the purpose of a simple sentence is not necessarily to do that.
Step-by-step explanation:
The question asks for "best"... so that implies ONE answer (thus, #2).
I think it is poorly-worded question.