💡 Detailed Explanation
The correct choice is 'いけ' (forming '〜てはいけません' - must not do). The grammar pattern '〜てはいけない / 〜てはいけません' indicates prohibition. Sentence translation: 'You must not speak in a loud voice inside the library.' 'なりません' would require the particle 'な' (e.g. '話さなければなりません' - must speak), but with '〜ては', it is '〜てはいけません'.