Option (2) is the correct choice to change the assertive sentence into an exclamatory sentence for the following reasons:

In exclamatory sentences, we use words or phrases that express strong emotions, excitement, or surprise. In this case, the sentence is about the delicious flavor of the mangoes, which is a subject that can evoke enthusiasm.

(1) "A delicious flavor these Mangoes have!" is not the most natural word order for an exclamatory sentence in English.

(2) "What a delicious flavor these Mangoes have!" is the appropriate choice because it uses the "What a" construction, which is common in forming exclamatory sentences. It expresses the speaker's excitement about the delicious flavor.

(3) "Mangoes have a delicious flavor!" is a declarative sentence and does not convey the same level of excitement as an exclamatory sentence.

(4) "These Mangoes have flavor delicious!" does not follow the typical word order for exclamatory sentences in English and sounds less natural.