An/A' is used as an indefinite article for a thing or person when the reader doesn't know which one we are referring to out of many nouns. It states the random nature of the subject.
'The' is used as a definite article for a thing or person when the reader knows which one we are exactly referring to. It states the specific nature of the subject.
Option D is correct as we're speaking of random mangoes,i.e. they're not specific and are more than one mango. Option C is incorrect as 'An' is used as the article for words with a vowel sound. Option A is incorrect as using the definite article won't suit the meaning of the sentence. because 'the mangoes' would imply specific but we don't know which mangoes. Option B is incorrect as 'A mangoes' sounds illogical and wouldn't provide meaning to the sentence.