When particle motion is parallel to wave propagation direction it defines longitudinal waves, so option A is correct.

The displacement at a particular instant is called instantaneous displacement, not mean or average which are different concepts.

Wave speed = f × λ = 1000 × 0.1 = 100 m s^-1; time = distance/speed = 2500/100 = 25 s.

In the accelerating frame the bob is in equilibrium under effective horizontal acceleration a and gravity g, so tan θ = a/g and θ = tan^-1(a/g).

Medical sonography uses ultrasonic waves with frequencies greater than 20 kHz, so 'greater than 20 kHz' is correct.

For small amplitudes, the period of a simple pendulum depends on its length and g, not mass or thread thickness.

Kinetic energy is proportional to velocity squared and thus varies as x^2 (or cos^2), giving a period half that of displacement, T/2.

At a given temperature, the speed of sound in an ideal gas is essentially independent of pressure and depends primarily on temperature and molecular properties.

Total mechanical energy in SHM is proportional to kA^2/2, so it scales with the square of the amplitude.

For displacement, the closed end of an organ pipe is a node (zero displacement) for standing waves; pressure behavior differs.