Translation:The bank becomes the largest private institution.
You'll see more examples here: http://michaelis.uol.com.br/moderno/ingles/index.php?lingua=portugues-ingles&palavra=passar
My dictionary says that "passar" can be translated as " happens". I have looked up the phrase "to come to be " or "to become" and "passar" is not listed. Moreover the hint was "spread". It this is an idiomatic phrase then it should have been introduced earlier. This type of stupid mistake just detracts from what is otherwise a very good program.
For this exercise it is not just the word, passar but the phrase, passar a ser which Paulenrique noted elsewhere works out as, turn out to be* (an English phrase) that can often be represented by "become".
You can switch "turn(s) out to be" with the given translations in the examples on the above linked page and see for yourself that they work out most of the time. Pretty nifty. :)
I daresay this could be used with, uma larva passa a ser uma borboleta.
As for when to introduce the phrase to us, I am not sure where or when it could be better positioned as it seems that it would draw complaints no matter where it gets put by those who feel duped by it.
One of the tenants of CNN as an international news organization when it started was to avoid the use of foreign[er] because that becomes centric to a particular country. So they might have used, "language learners* instead.
However, I have thought the same as you as I struggle with the 57 different PT possibilities but, when I really look at it, all those auxiliary verb words in English to change the meanings do not seem all that different from verb endings in other languages. Plus spread is just one of many irregular (abby-normal) English verbs to keep in mind. :D
Spread, spreads, spreading, has, have, had, will, will be, will have, will have been, would be, would have, would have been, was, did, does, let's, to, and finally the participles, spreading, spread, having spread.
It's enough to make my head spin, and I haven't even touched on bread-spread versus bedspread. =)
Plus my Portuguese friend often brings up the English fondness for abbreviations (assn, adm, admin, admon, ad, advert, Aug, Jan, Feb, Cal, NY, Wisc., Abbr.), and acronyms (A/C, AC/DC, AD, BC, RSVP, USDA, FBI, CIA, CSI, CBS, NBC, CNB, CNN, CCN, BBC, DMV, DNC, RNC, POTUS, SCOTUS, SOS, SNAFU...).