Consider the following statements: Statement - I: When an ultrasound travels from one medium to another, its wavelength, frequency and speed change. Statement - II: When travelling from one medium to another, the frequency of the ultrasound wave remains almost unchanged. Statement - III: When travelling from one medium to another, the frequency and speed of the ultrasound wave change. Statement - IV: Echocardiography is a technique used in medical diagnostics which uses ultrasound. Which of the above combinations is correct?

Statement I: When a wave travels from one medium to another, its frequency remains constant because it is determined by the source generating the wave. Its speed does change depending on the properties of the new medium, and consequently, its wavelength changes (Speed = Frequency × Wavelength). So, the statement claiming frequency changes is incorrect. Statement II: When travelling from one medium to another, the frequency of the ultrasound wave remains almost unchanged. This is a correct statement; frequency is a conserved property of the wave determined by its source. Statement III: This statement incorrectly claims that both frequency and speed change. As explained, frequency remains constant while speed changes. Statement IV: Echocardiography is indeed a medical diagnostic technique that uses ultrasound waves to image the heart. This statement is correct.