Answer:
They should be complex.
Explanation:
Formal writings can be defined as those writings that are usually written to those audience that are not known personally or intimately. Formal writings are prominently used in acadmic writings, research papers, professional writings, etc. The tone of such writings are generally serious.
The structure of formal writings is complex. The sentences are ordinarily written in complex format, using long sentences, explaining and elaborating each points and topics.
Therefore, the correct asnwer is option C. They should be complex.