Which of the following waves cannot travel in a vacuum?

Transverse and longitudinal waves can propagate in solids,whereas only longitudinal waves can propagate in air. Explain this statement.

Mechanical waves on the surface of a liquid are

The figure shows wave fronts in still water moving in the direction of the arrow towards the interface $PQ$ between a shallow region and a deep (denser) region. Which of the lines shown may represent one of the wave fronts in the deep region?

$Assertion :$ When a beetle moves along the sand within a few tens of centimeters of a sand scorpion,the scorpion immediately turns towards the beetle and dashes towards it.
$Reason :$ When a beetle disturbs the sand,it sends pulses along the sand's surface. One set of pulses is longitudinal while the other set is transverse.

What are called mechanical waves?