The Lowest Usable Frequency for communications between two specific points✓Правильный ответ
The Lowest Usable Frequency for communications to any point outside a 100-mile radius
The Lowest Usable Frequency during a 24-hour period
The Lowest Usable Frequency during the past 60 minutes
Объяснение
LUF stands for Lowest Usable Frequency, and like the MUF it is defined for one specific path between two specific points, because absorption depends on how much sunlit D region the wave crosses. It is not a general figure for a radius or for a fixed time window.