Explanation

Sound waves with frequencies below 20 Hz are called infrasonic (subsonic) waves, not ultrasonic waves; 'ultrasonic' refers to frequencies above the audible range, i.e. above about 20,000 Hz. Hence statement (B) is not correct. The other statements are true: sound in gases propagates as longitudinal waves, louder sounds have larger amplitude, and high-frequency audible sounds are perceived as sharp or shrill. The incorrect statement is (B).