Genetic mutation can alter the sequence of a protein, changing the type of protein(s) found in a cell, and it could also change protein number if the mutation occurred in some important regulatory part of the gene (promoter, enhancer). Regulation of gene expression cannot change the identity of the proteins found, unless alternative splicing is involved, but it can readily alter the number of (copies of) proteins found in the cell.
Work Step by Step
Here, the key is to make a list of features/properties of both mutation and expression regulation. See which feature(s) affect protein identity and number--basically, mutation is much better at altering identity while regulation of expression more often alters number.