The main cause that tsunami waves lose some of their energy as they get closer to the shore is

Tsunami waves lose energy near the shore chiefly because there is less depth or room, forcing the water to pile up and slow.

Buildings on the beach are struck by the wave but are not the main reason it loses energy first.

Friction and turbulence from the beach add some loss but are secondary to the shallowing effect.

Since reduced depth is the main cause, none of the above does not apply.

As depth falls the wave slows, shortens and grows taller, converting speed into height.

This shoaling turns a low open-sea swell into the towering wall that reaches the coast.