💡 Detailed Explanation
The prefix '未' (mi-) means 'not yet'. '未解決' (mikaiketsu) means 'unresolved' or 'not yet solved'. The other options do not fit: '無' (mu-) means 'without' or 'non-existent', '不' (fu-) implies 'lacking' or 'negative', and '非' (hi-) means 'anti-' or 'un-'.