The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
Mosquitoes are known to be the vectors of diseases like malaria and dengue fever caused by Plasmodium and Dengue virus. Earlier the mosquito-population had more DDT-sensitive but less DDT-resistant mosquitoes. When DDT was not being used, the DDT-resistant remained dominated by DDT-sensitive mosquitoes. But when the use of DDT as an insecticide started (introduced in 1940s), DDT-resistant mosquitoes had a competitive advantage over their counterparts. Their DDT-resistant property spread over more and more members of the population so now the mosquito population is dominated by DDT-resistant ones. It is also an example of directional selection. Thus according to principle of Natural Selection, chemical insecticides would remain useful only for a limited time. So, both Assertion and Reason are correct and Reason is the correct explanation for Assertion.