The intensity of an ultrasound beam is measured using a hydrophone. A hydrophone is a specialized device that detects and measures the acoustic pressure of the ultrasound waves in water or tissue-mimicking materials. It is highly sensitive and can measure the variations in pressure, which are used to calculate the intensity and other acoustic parameters of the ultrasound beam.
ARDMS Sonography Principles and Instrumentation guidelines
Hoskins, P. R., Thrush, A., Martin, K., & Whittingham, T. A. (2010). Diagnostic Ultrasound: Physics and Equipment.

Axial resolution refers to the ability to distinguish two structures that are close to each other along the direction of the sound beam. It is determined by the spatial pulse length (SPL), which is the product of the number of cycles in a pulse and the wavelength. Shorter SPL means shorter pulse duration, which leads to better axial resolution. This is because shorter pulses allow for better separation of echoes from closely spaced structures.
ARDMS Sonography Principles and Instrumentation guidelines
Kremkau, F. W. (2015). Diagnostic Ultrasound: Principles and Instruments.

Comprehensive and Detailed Explanation From Exact Extract:
Persistence averages multiple frames to smooth out random noise fluctuations, reducing the appearance of noise in the image and improving image quality.
According to sonography instrumentation reference:
"Persistence reduces noise by averaging data from consecutive frames, improving signal-to-noise ratio and producing a smoother image." Therefore, the correct answer is D: Increase persistence.
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Comprehensive and Detailed Explanation From Exact Extract:
A tissue-mimicking phantom contains small targets at known separations to evaluate the system's axial resolution, lateral resolution, depth calibration, and more.
Principles and Instrumentation state:
"Tissue-mimicking phantoms assess spatial resolution (axial, lateral), depth accuracy, and penetration performance."
* Hydrophones (B) measure acoustic pressure.
* Beam profilers (C) assess beam characteristics.
* Doppler phantoms (D) test Doppler function.
Therefore, the correct answer is A: Tissue-mimicking phantom.
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When sound waves encounter a smooth, flat surface, the primary interaction is reflection. This means the sound waves bounce back towards the transducer, producing echoes that are used to create the ultrasound image. Reflection is most effective when the surface is smooth and perpendicular to the sound beam, allowing for the maximum return of sound waves to the transducer.
ARDMS Sonography Principles and Instrumentation guidelines
Zagzebski, J. A. (1996). Essentials of Ultrasound Physics.