The impedance at the centre of a dipole antenna more than 3 wavelengths above ground would be nearest to:

The feed-point impedance of a standard half-wave dipole antenna in "free space" (far away from the influence of the ground or other objects) is approximately 73 ohms. When an antenna is placed very close to the Earth, ground reflections interact with the antenna and can significantly alter this impedance. However, once a dipole is elevated to more than three wavelengths above the ground, the ground's effect becomes negligible. At this height, the antenna behaves as if it is in free space, making its centre impedance settle near its theoretical free-space value, which is closest to the 75-ohm option provided.