Select the incorrect statement
Option
A. The frequency of a wave depends on its source and not on the medium
B. Only a transverse wave can be polarised
C. For the propagation of mechanical wave the medium must be least elastic
D. The mechanical waves are always longitudinal as liquids and gases cannot sustain shear

Hint: A wave is a propagating mechanical disruption of one or more quantities in physics, mathematics, and related fields, often represented by a wave equation. At least two field concentrations in the wave medium are present in physical waves.

Complete step-by-step answer:
In physics, frequency refers to the number of crests that pass through a fixed point in the medium in a given amount of time. As a result, it should be determined by the source rather than the medium. If I have a source that vibrates faster than yours, my source's amount of crests per second (for example) would be greater than yours. However, the wave's speed is determined by the medium's properties, such as temperature and density. We also know that wavelength is dependent on the speed of the wave (i.e. the medium) and the frequency, so it varies depending on the medium. Consider frequency to be a constant in the equation since it is solely dependent on the source— so if the speed varies, frequency remains constant (i.e. medium changes).
Since their vibration is perpendicular to the direction of energy travel, only transverse waves can be plane polarised; this vibration can also occur at various points. Since the direction of vibration and propagation are the same, longitudinal waves cannot be plane polarised. In a gas or liquid, acoustic waves (sound waves) do not have polarisation, but all electromagnetic waves do.
The following are the essential properties of a medium for mechanical wave propagation:
I) Elasticity: The medium must have elasticity in order for the particles to return to their normal locations after being disrupted.
(ii) Inertia: In order for particles to store kinetic energy, the medium must have inertia or mass.
The medium can have the most elasticity such that as the particles are moved, they return to their original locations.
The essence of the medium and mode of excitation determine whether a mechanical wave is transverse or longitudinal. Mechanical waves in gases and liquids, such as sound waves in air or water, are often longitudinal. This is due to the fact that fluids cannot withstand shear.

So, the correct answer is “Option C”.

Note: Longitudinal waves are waves in which the medium's vibration is relative to the wave's travel path and the medium's motion is in the same (or opposite) direction as the wave's propagation. Mechanical longitudinal waves are also known as compressional or compression waves because they induce compression and rarefaction as they pass through a medium, as well as pressure waves because they cause pressure changes. Longitudinal waves are how sound flows. Sound flows in a 90-degree transversal pulse.