Why is the answer "tę zupę" and not "ta zupa"? My understanding is that the "tę" is being used in the accusative case -- that is, it's referencing the object. That being said, if we switched this sentence around to say "This soup is the soup she likes", would we start the sentence with "Ta zupa"?
Or one could also say: "Ta zupa jest lubianą przez nią zupą" - This soup is the soup (that's) being liked by her.... And so on. But come on, such construction would be found only in some public speeches or in literature. :)
And generally - indeed - in this case we have subject + verb + direct object (ta zupa), thus we must alter it into tę zupę.
Here is the full chart of declension for smaczny. Under "declension of smaczny"