Dolphin echolocation is similar to ultrasound. Reflected sound waves
allow a dolphin to form an image of the object that reflected the waves.
Dolphins can produce sound waves with frequencies ranging from
0.25 kHz to 220 kHz, but only those at the upper end of this spectrum
are used in echolocation. Explain why high-frequency waves work better
than low-frequency waves.

Waves with high frequencies have shorter wavelengths that work better  than low frequency waves for successful echolocation.

Explanation:

To understand why high-frequency waves work better  than low frequency waves for successful echolocation, first we have to understand the relation between frequency and wavelength.

The relation between frequency and wavelength is given by

λ = c/f

Where λ is wavelength, c is the speed of light and f is the frequency.

Since the speed of light is constant, the wavelength and frequency are inversely related.

So that means high frequency waves have shorter wavelengths, which is the very reason for the successful echolocation because waves having shorter wavelength are more likely to reach and hit the target and then reflect back to the dolphin to form an image of the object.

Thus, waves with high frequencies have shorter wavelengths that work better  than low frequency waves for successful echolocation.

the displacement produced by the body.  

the amount of force subjected to the body.  

the angle between the direction of force and displacement