The key here is to remember is that when a certain number DO retire, that will make the ABSOLUTE number of total employees less--that is what is throwing you. So take each of the answer choices and subtract them from the original number , 560 long-term employees (that is 70% of the original 800). Ah...

A SET OF PEOPLE HAVE ARRIVED sounds right because you are confused about the SUBJECT!! What is throwing you--and what you have NOT obviously learned--is the ANY word in a PREPOSITIONAL PHRASE ("of people") CANNOT be the subject of a sentence in English. The subject of this sentence is SET ...