'jabłka' is 'not masculine-personal' plural. The 'not masculine-personal' form of the adjective in Nominative is identical to neuter singular. So "smaczne".
We think that "pyszne" = "delicious" and is stronger than just "tasty", but there were so many reports for "delicious" that we decided to give up on this distinction.
Apples are simply the subject of the sentence here, so they take Nominative, which is "jabłka". "jabłek" is Genitive and makes absolutely no sense here.