To propagate both longitudinal and transverse waves,a material must have

Given below are some examples of wave motion. State in each case if the wave motion is transverse,longitudinal or a combination of both:
$(a)$ Motion of a kink in a longitudinal spring produced by displacing one end of the spring sideways.
$(b)$ Waves produced in a cylinder containing a liquid by moving its piston back and forth.
$(c)$ Waves produced by a motorboat sailing in water.
$(d)$ Ultrasonic waves in air produced by a vibrating quartz crystal.

The waves in which the particles of the medium vibrate in a direction perpendicular to the direction of wave motion are known as:

What is called a transverse wave? And what is called a longitudinal wave?

What are called mechanical waves?

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?