Sound is a mechanical wave that requires a material medium to travel. It can propagate through solids, liquids, and gases because these media possess the necessary elasticity and inertia to transmit the mechanical vibrations from particle to particle. Sound cannot travel through a vacuum.

Audible waves fall within the range of 20 Hz to 20,000 Hz (a-2). Infrasonic waves are those with frequencies below 20 Hz (b-1). Ultrasonic waves are those with frequencies above 20,000 Hz (c-3). Therefore, the correct matching is a-2, b-1, and c-3.

Sound travels significantly faster in liquids and solids than in gases because the molecules are more closely packed, allowing for more efficient transmission of mechanical vibrations. In water, the speed of sound is approximately 1,480 m/s, which is roughly 4.3 to 5 times faster than the speed of sound in air at room temperature.

Sound waves travel through a medium as longitudinal waves. These waves consist of alternating regions of high pressure and density, known as compressions, and regions of low pressure and density, known as rarefactions. A compression represents the phase of the wave where particles are packed most closely together, effectively increasing the local density of the medium.

Pitch is the human perception of the frequency of a sound wave. A higher frequency corresponds to a higher pitch, while a lower frequency corresponds to a lower pitch. Amplitude is related to the loudness of the sound, not the pitch.

An echo is the repetition of a sound caused by the reflection of sound waves from a surface. When sound waves encounter multiple surfaces or obstacles, they can reflect back to the listener at different intervals, resulting in multiple echoes or reverberation. This phenomenon relies entirely on the reflection of sound waves off hard surfaces.

Sound is a mechanical wave that requires a material medium, such as air, water, or solids, to propagate. Since the moon lacks an atmosphere and is essentially a vacuum, there are no molecules to transmit the vibrations from the sound source to an observer's ear, making sound propagation impossible.

The speed of sound in air depends on temperature. At 20°C, the speed of sound is approximately 343 m/s. This value is widely accepted as the standard reference speed for sound propagation in dry air at room temperature.

The speed of a wave (v) is calculated using the formula v = f × λ, where f is the frequency and λ is the wavelength. Given f = 600 Hz and λ = 3 m, the speed is 600 × 3 = 1800 m/s, which is expressed as 1800 ms-1.

The human brain retains the sensation of sound for approximately 0.1 seconds, a phenomenon known as persistence of hearing. For an echo to be perceived as a separate sound from the original, the reflected sound must reach the ear at least 0.1 seconds after the original sound has ceased.