What unit is used to express relative intensity or noise level?

The correct choice is decibels, which is a logarithmic unit used to measure sound intensity and levels of noise. The decibel scale quantifies sound intensity and allows for easy comparison of sound levels, given that sound can vary significantly in intensity. A change of 10 decibels typically represents a tenfold increase in intensity, making it an effective scale for expressing both quiet and loud noises on a manageable range.

The other options listed do not serve the same purpose as decibels. Echoes refer to the reflection of sound that is heard after a delay, reflections relate to how light or sound bounces off surfaces, and wavelengths refer to the distance between successive peaks of a wave, which is a measure of frequency rather than intensity. Each of these terms is relevant in different contexts of sound and wave behavior but does not specifically address the measurement of sound intensity or noise levels as decibels does.