The correct option is A have been
Let’s examine the sentence: “A music and dance show have been organised to raise funds for the orphanage on the first Saturday of next month.” The subject of the sentence, ‘music and dance show,’ is singular as the noun 'show,' and not 'shows,' is used here. So, it requires a verb that is used for singular nouns. "Have" is the root verb for present tense and is generally used alongside the pronouns, I / You / We / Ye and They, and plural nouns. "Has" is the verb used alongside the pronouns, He / She / It and Who, and singular nouns. As the noun phrase or subject of the sentence is singular, it requires the verb ‘has’ and not ‘have.’ Hence, the word or phrase that must be changed for the sentence to be correct is indicated by Option A. So, Option A is the correct answer.