Find the synonym of the underlined word. 

This matter is totally irrelevant to the discussion we have just had. 

'Irrelevant' means unrelated to or unrelated to something. "an irrelevant comment"

"This matter is completely irrelevant to the debate we just had," indicates immaterial or foreign in this context. As a result, choice B is correct, whereas the other options are incorrect because they do not yield the correct answer.

A person who resides in another nation but does not hold citizenship there is considered to be an alien. An example of an immigrant is a US citizen who resides in Italy without having Italian citizenship.
A suitable person or object is one that is appropriate or acceptable in a specific circumstance. An appropriate item would be anything like a great attire for a formal dinner.