For fitness to be unaffected, mutations need to occur where natural selection cannot act on them: in DNA which does not affect phenotype. These places include the various kinds of noncoding DNA which do not affect phenotype, such as DNA between genes which does not alter gene expression. Also, some mutations are silent, not changing protein sequence/fuction since they do not change the amino acid sequence due to wobble in the genetic code.
Work Step by Step
See Concept 17.5 to review the basics of mutation, including the various kinds which occur. Also review the genetic code and wobble. Make a list of DNA features which do not affect gene expression/phenotype-- the mutations which don't affect fitness should fall into those areas. To have no effect on fitness, no phenotypic or otherwise functional change should be observed. Any genetic changes that would not produce such a corresponding change could occur without altering fitness.