The aperture of a transducer is the active area that emits and receives the ultrasound waves. In a linear array transducer, increasing the aperture (using more elements for transmission and reception) results in a deeper focus because the beam is more tightly focused over a longer distance. This improves lateral resolution at greater depths, as the ultrasound beam maintains a narrower width for a longer distance. It allows for better imaging of deeper structures without sacrificing resolution.
American Registry for Diagnostic Medical Sonography (ARDMS). Sonography Principles and Instrumentation (SPI) Examination Review Guide.

Sensitivity is the term for an ultrasound system's ability to display low-level echoes. It refers to the system's capacity to detect and accurately display weak echoes returning from tissues. High sensitivity allows the sonographer to visualize structures that produce faint echoes, such as small or low-contrast lesions. This parameter is critical for ensuring that subtle pathological changes are not missed during imaging. Reference:
ARDMS Sonography Principles and Instrumentation guidelines
"Sonography: Principles and Instruments" by Joan P. Baker and Marveen Craig

Enhancement is an artifact that results from decreased attenuation. When an ultrasound wave travels through a medium with lower attenuation compared to surrounding tissues, it loses less energy. Consequently, the structures located deeper than the low-attenuation medium appear brighter on the ultrasound image. This artifact is commonly observed behind fluid-filled structures, such as cysts or the urinary bladder, where the sound waves encounter minimal resistance and thus less attenuation. Reference:
ARDMS Sonography Principles and Instrumentation guidelines
"Diagnostic Ultrasound: Principles and Instruments" by Frederick W. Kremkau

Comprehensive and Detailed Explanation From Exact Extract:
Damping reduces the number of cycles per pulse, shortening pulse duration and spatial pulse length, which directly improves axial resolution.
Principles and Instrumentation:
"Axial resolution improves with shorter spatial pulse length, achieved by damping, which limits ringing of the transducer."
* Lateral resolution (A) depends on beam width.
* Contrast resolution (B) relates to dynamic range.
* Temporal resolution (C) is tied to frame rate.
Therefore, the correct answer is D: Axial.

The color bar on a Doppler ultrasound display indicates the range of velocities that the system can detect and display. In color bar A, the scale is set to a higher maximum velocity (64 cm/s), while in color bar B, the scale is set to a lower maximum velocity (16 cm/s). Adjusting the scale (or velocity range) changes the upper and lower limits of the velocities displayed, which affects the sensitivity of the Doppler system to detect flow velocities. Lowering the scale allows for better visualization of lower velocities, but it may also increase the likelihood of aliasing for higher velocities.
Reference:
American Registry for Diagnostic Medical Sonography (ARDMS). Sonography Principles and Instrumentation (SPI) Examination Review Guide.