💡 Detailed Explanation
The correct answer is 「納得」 (nattoku), which means 'consent', 'assent', or 'understanding/acceptance'. The phrase 「納得がいかない」 is a set idiom meaning 'cannot accept/does not make sense'. 「理解」 (rikai) means 'comprehension' (usually 「理解できない」). 「承知」 (shōchi) means 'acknowledgment/agreement'.