What factors affect the MUF?

The Maximum Usable Frequency (MUF) is the highest frequency that will support reliable skywave communication between two specific points on Earth. Because it relies heavily on the ionosphere's refractive capabilities, it is highly dynamic. Solar radiation is responsible for creating the ionosphere, meaning that solar flares, sunspots, time of day, and seasonal changes all directly dictate how much ionization is present. Higher ionization levels generally support a higher MUF. Additionally, the MUF is entirely dependent on the geometry of the communication path. The exact distance between the transmitting and receiving stations, along with their geographic locations, dictates the angle at which the signal strikes the ionosphere. Because all these factors play a vital role, all the listed choices are correct.