After a mutation at a genetic locus, character of an organism changes due to change in

As per the central dogma, the DNA makes RNA and RNA makes proteins. So, the code in DNA determines the code in mRNA and the code in mRNA determines the sequence of amino acids. The amino acid chains are the basis of protein structure. So, if something goes wrong in the DNA (like mutation in the genetic locus), then the mRNA will also be wrong and if mRNA is wrong, then the amino acid chain will be wrong and will cause a problem in the protein. The changes in the protein leads to changes in certain characters of an organism.
Thus, the correct answer is option (A), 'Protein structure'.