Answer:
unless
Step-by-step explanation:
In English language, a conjunction can be defined as an English word that is typically used for connecting two or more clauses, words, phrases or sentences. Some examples of conjunction are but, and, although, because, or, yet, unless, so, however, nor, etc.
Unless is synonymous with "except if" or "if not" and as such it's a conditional statement. Thus, it's used to express an action that will or will not happen if something else isn't going to happen or not true (false).
In this scenario, staff members are instructed not to use the executive restroom, if they've not obtained a written authorization.
Hence, the complete sentence would be written as follows;
"All staff members are reminded that you should not use the executive restroom unless you have written authorization to do so."