which possible mutation of the DNA triplet GAC will affect the production of a protein by the gene that contains this triplet

To answer this question we need to look at the genetic code to see what amino acid GAC triplet codes.

As is in DNA we need to pass it to RNA

GAC = CUG = Leu

Now we have four possible options, so we need to obtain the RNA triplet and look at the genetic code so we can see what each code.

A.- ATC = UAG = Stop

B.- AAC = UUG = Leu

C.- GAT = CUA = Leu

D.- GAA = CUU = Leu

So options B to D all code for leucine, the same amino acid as the original triplet, meanwhile option A does not code for another triplet but is a Stop codon, meaning that protein production will stop there, so the correct answer is option A as a premature stop definitely will leave the protein unfinished and with no function or defective.