Explanation:

The word 'न्यून' is formed by the combination of 'नि' + 'ऊन'. In this combination, the short 'इ' sound from 'नि' combines with the long 'ऊ' sound from 'ऊन' to form 'यू'. This transformation, where 'इ/ई' + dissimilar vowel becomes 'य', 'उ/ऊ' + dissimilar vowel becomes 'व', and 'ऋ' + dissimilar vowel becomes 'र', is characteristic of 'यण् संधि' (Yan Sandhi). Options A (Ayadi), C (Dirgh), and D (Guna) involve different vowel combination rules.