I think that "Which pizza is tasty?" should also be a correct answer as tasty and delicious are synonyms
"Tasty" is accepted in later lessons. Actually, the most natural English expression would be "Which pizza is good?" or "Which pizza tastes good?" In America we rarely insert "delicious" into a question, unless looking for confirmation of an opinion--"Isn't it delicious?" or "It's delicious, isn't it?"