Answer:
A feature that should always be included in an example of formal writing is the following one:
C. Correct grammar.
Step-by-step explanation:
In order to express oneself in a formal way, correct grammar must be used, since it makes for a serious tone, that is, it shows whatever was written was done so carefully and seriously, as a sign of respect for the addressee. In formal writing, even abbreviations must be avoided, as well as contractions. Therefore, correct grammar is the way to go if you want to achieve a formal tone.