The critical day length of Hyoscyamus for flowering is :

Explanation

Hyoscyamus, like many plants, has a specific photoperiod requirement known as the critical day length, which is the minimum daylight hours necessary to trigger flowering. For Hyoscyamus, this is 15.5 hours. If the day length is less, the plant will not flower. The other options provide shorter day lengths, which are insufficient to meet the plant's photoperiodic needs. Understanding photoperiodism is crucial in botany, as it significantly influences plant development and reproductive cycles.