When a mutation results in no significant change in the production of a functional protein, it's called a?

The right answer is: neutral mutation.

Neutral mutation is a nucleotide change in the DNA of the leading to the replacement of one amino acid by another amino acid possessing the same physicochemical properties.

    example: AAA to AGA (Lysine to Arginine).

both these two amino acids are positively charged, so the charge of the protein will not change