The Minimum Usable Frequency for communications between two points
The Maximum Usable Frequency for communications between two points✓Correct answer
The Minimum Usable Frequency during a 24-hour period
The Maximum Usable Frequency during a 24-hour period
Explanation
MUF stands for Maximum Usable Frequency, defined for communication between two specific points, since the maximum depends on the angle at which the wave strikes the layer along that particular path. A steeper path returns a lower maximum frequency than a long shallow one.