What does maximum usable frequency mean?

The Maximum Usable Frequency (MUF) is defined as the highest radio frequency that can be successfully used for communication between two specific points via ionospheric refraction. As frequency increases, the ionosphere bends the radio signal less. At the MUF, the bending is just enough to return the signal to the target destination. If you transmit at a frequency above the MUF, the signal will not bend enough and will escape right through the ionosphere into space. Options mentioning the lowest frequency describe the Lowest Usable Frequency (LUF), which is determined by signal absorption in the D layer rather than refraction.