Why is mutation beneficial to the process of evoloution?

Many mutations do produce changes in phenotype,It helps the bodies of the species to re-adapt to more locations and habitats.

Im assuming this is a multiple choice answer what is the answers to chooce from?

You should never redesign your experiment. try and see what in your experiment you made a mistake on or try and test your experimemt with somebody else. see if they get the right results.