Option (3) is correct. 

The length of vocal chords of women is about 15 mm.

When we pluck the strings of a guitar, the only sound we hear is the sounds of the vibrating strings as well as the sound of the vibrating body.

Sound can travel through all solids, liquids or gases. Hence, option (4) is false.

Vocal chords in human beings vibrate to produce sound.

Above 80 dB, a sound becomes physically painful.

Sound enters the outer ear, travel downs the ear canal and strikes the ear drum. The ear drum sends the vibrations to the inner ear, from where the signal goes to the brain. Hence, option (2) is correct.

The loudness of a sound depends upon its amplitude. The higher the amplitude, the louder is the sound and vice-versa. The pitch or shrillness of a sound is determined by its frequency. The higher the frequency, the shriller is the sound and vice-versa. Hence, option (4) is false. A feeble sound will have a low amplitude, not a high amplitude. 

Ultrasound equipment works at frequencies not less than 20000 Hz.

The loudness of a sound depends upon its amplitude. The higher the amplitude, the louder the sound and vice-versa. The pitch or shrillness of a sound is determined by its frequency. The higher the frequency, the shriller the sound and vice-versa. Hence, option (4) is correct.

Loudness of a sound is proportional to the square of its amplitude. Hence, let amplitude of the sound of 20 dB be a. Then, 20 = ka²; Sound of 80 dB = kb² (where b is the amplitude of sound at B). i.e. 4ka² = kb² i.e. k(2a)² = kb² Hence, b = 2a. Thus, amplitude at B is double of that at A.

Plants cut down the sounds reaching the residents. Thus, they reduce the harmful effects of noise pollution.

Frequency is measured as the number of oscillations per second. Hence, option (2) is correct.

Ektara is a string instrument. Hence, it cannot be played by simply striking it.

The to and fro, or back and forth motion of an object is termed as vibration.