The correct option is D Rarely useful
Mutations are the changes in chemical structure of genes which are manifested in altered protein structure and function. Mutation in a gene renders a protein less functional or non-functional; thus they are mostly harmful, for example, the point mutation in beta-globin gene render haemoglobin less functional; this makes option A incorrect. Mutations are sometimes useful as the altered protein structure and function imparts beneficial phenotype; for example, the same mutation in $$\beta$$-globin gene impart malaria resistance; they are rarely useful which makes option C incorrect and option D correct. The correct answer is D.