Resonance refers to an effect in which the
a. whole instrument vibrates on its natural or fundamental frequency.
b. intensity of a sound decreases over time.
c. pitch of a note is compared to a pure tone.
d. pitch changes when two waves with similar frequencies interfere with each other.

a.       whole instrument vibrates on its natural or fundamental frequency. Resonance refers to manner wherein the whole instrument vibrates on its natural or fundamental frequency especially when being forced by a second object through a  vibrational movement. It is a Latin word which means to resound.  Most sound productions are caused by resonance or sounding together loudly. That strange sound you can hear from a shell when placed near your ear is among the best example that can be explained through resonance. Musical instruments such as woodwinds and brass operates through resonance.

Not b or c so that leaves you with a or d. a is the better choice.