In the following question, out of the four alternatives, select the alternative which is the best substitute of the phrase.
An unintended consequence of an event or action, especially an unwelcome one.

Let's look at the definitions of the choices:
repercussion: An unintended consequence of an event or action, especially an unwelcome one. This matches the phrase completely.

reverberation: A prolongation of a sound; resonance, or a continuing effect.

backlash: A strong negative reaction by a large number of people, especially to a social or political development.

flak: Strong criticism.

Hence, repercussion is the best substitute.