"The play was so popular that the theater was almost full."
Translation:Lo spettacolo era talmente popolare che il teatro era quasi pieno.
"cosi'" can be and adverb or a conjunction. When it is an adverb it can have two meanings. It can mean:
(in) this way, like this, like that "Ho scritto cosi' (I wrote like that)" "Non ho detto cosi' (I didn't say that)"
so, so much "E' cosi' gentile (he is so kind)" In this case "cosi'", "talmente" and "tanto" are interchangeable. The DL sentence can be written as "Lo spettacolo era cosi'/talmente/tanto popolare che il teatro era quasi pieno"
So the hints give three different translations for "so" but if you use any one of them, you are marked incorrect. "talmente" was not even given. It is another case of DuoLingo just wanting to prove it can really annoy us by giving us words it will not accept. Very frustrating!!