Which of the following may cause excessive harmonics in a transmitter?

Harmonics are multiples of the fundamental operating frequency. They are primarily generated when an amplifier stage operates non-linearly, which frequently happens when it is overdriven. Driving a stage with too much input signal pushes the amplifier into clipping or saturation. This distortion alters the shape of the waveform from a pure sine wave, effectively generating unwanted harmonic frequencies. Conversely, things like a non-resonant antenna or an internal tuner malfunction might cause high SWR and reflect power, but they do not actively create harmonics.