Which of the Following Is an Example of a Rule

Which of the following is an example of a rule?

A) a general mandatory guideline asking all employees to leave their work machines in good order
B) a statement issued by the company specifying the sales projection for the next fiscal year
C) an informal code of conduct recommending that employees help each other if time permits
D) a recommendation by the HR department that employees wear formal clothes during the week
E) a suggestion by the technical department to save all work-related files on D drive