Lethal mutations are the one that renders the organisms with reduced or no reproductive ability and thus are harmful while only useful heritable traits that impart longevity and increased productivity are selected for crop improvement in agriculture. Naturally occurring mutations are termed as spontaneous mutations while the intentional induction of mutations for a desired trait in selected genotypes refers to as artificially induced mutations. Since the rate of naturally occurring spontaneous mutations is very low; artificially induced mutations for desired traits speed up the process of breeding program for crop improvement. Thus, the correct answer is option B.