Which of the following mutations potentially has the greatest effect on gene expression?

A mutation in the promoter will alter the initiation of the translation so impact the expression from that gene.

Introns are spliced out and a third base alteration in the codon will often code for the same or a similar amino acid because of the degeneracy of the code and code families of amino acids. The degenerate code refers to 64 codons exist for 20 amino acids therefore multiple codons result in the same amino acid. Amino acids, belonging to a single biosynthetic family, have codons that are contiguous in the genetic code so a third base change in an acid polar base will code for another acid polar base so the protein will retain function usually.

A change in the 3' untranslated sequence that follows the gene's reading frame stop codon on the mRNA will not change the genes expression.