In the original sentence "These mangoes have a delicious flavour," the speaker is making a statement about the delicious flavor of the mangoes. To convert it into an exclamatory sentence, expressing strong emotion or surprise, the structure "What a delicious flavor these mangoes have!" is used.

In exclamatory sentences, "What" is often followed by "a" and an adjective, creating a pattern that conveys a sense of wonder, surprise, or admiration. In this case, it emphasizes the speaker's positive reaction to the delicious flavor of the mangoes.