The fruit fly Drosophila melanogaster has an allele that confers resistance to DDT and similar insecticides. Laboratory strains of D. melanogaster have been established from flies collected in the wild in the 1930s (before the widespread use of insecticides) and the 1960s (after 20 years of DDT use). Lab strains established in the 1930s have no alleles for DDT resistance. In lab strains established in the 1960s, the frequency of the DDT-resistance allele is 37%. Which statement is correct?a. Alleles for DDT resistance arose by mutation during the period of DDT use because of selection for pesticide resistance.
b. The evolutionary fitness associated with the heritable trait of DDT resistance changed once DDT use became widespread.
c. Resistance to DDT evolved in some fruit flies in order to allow them to survive.

Option B, The evolutionary fitness associated with the heritable trait of DDT resistance changed once DDT use became widespread.

Explanation:

As per Darwin’s theory, few traits are inherited from the parents and some are acquired from the environment. Offspring can inherit genetic traits such as skin color, height, weight, eye color etc. from parent but the traits which offspring adopt as a response to their interaction with environment is specific to the individual only and is passed on if only it mutate the genes and is not behavioral in nature.  

Resistance to DDT is not a behavioral adaptation to environment. Hence,  the gene responsible for developing resistance against DDT fruit fly Drosophila melanogaster evolved with the increased usage of DDT. Genes which help offspring to survive in the persisting environment are passed on by the parents. Thus, in 1960 several fruit fly Drosophila melanogaster species had DDT resistant gene.

Option B, is correct