Fill in the blanks with the most appropriate option:-

That mission is so __________ that it must not be discussed outside anywhere.

confidential

The correct option to fill in the blanks is confidential.

The sentence "That mission is so confidential that it must not be discussed outside anywhere" means that the mission is secret and should not be shared with anyone outside the organization. The other options are not appropriate because they do not convey the same meaning.

• The word "unique" means being the only one of its kind. The mission is not unique, as there are many other missions that are also confidential.
• The word "common" means being shared by many people. The mission is not common, as it is only known to a few people.
• The word "regular" means happening or done on a regular basis. The mission is not regular, as it is a one-time event.