I found the exercise on the internet and here are the options:
"the low friction of waves at the shoreline
the high friction of waves at the shoreline
the high amplitude of the waves at origination
the long wavelength of the waves at origination"
The tsunami property that most contributes to the flooding is "the long wavelength of the waves at origination". The long wavelength that the tsunamis have at their origin makes so that when the wave reaches the shore, instead of a normal spreading, a much longer spreading takes place taking the ocean as far as the wavelength allows.