Assertion: DDT has now become almost ineffective against mosquitoes.
Reason: Introduction of DDT brought a change in the gene frequency with a shift from DDT-sensitive to DDT-resistant mosquitoes.
A. Both assertion and reason are correct and the reason is the correct explanation for the assertion.
B. Both assertion and reason are correct and the reason is not the correct explanation for the assertion.
C. Assertion is correct but the reason is incorrect.
D. Assertion is incorrect but the reason is correct.

Hint: DDT was the very first chemical insecticide to be established. It was used with significant success to eradicate malaria, typhus as well as other insect-borne human diseases. It's been controversial for its environmental consequences.

Complete answer:
DDT-resistant mosquitoes have grown at a rapid pace that if a local spray project is underway, several mosquitoes would become resistant in a matter of a few months instead of years. DDT has become ineffective so early now that DDT-resistant mosquitoes occur at low frequency in the worldwide mosquito population and, when a native community is sprayed, a powerful selection effect is instantly generated in pursuit of resistant insects. It's just a matter of time until the mosquitoes invaded. DDT-resistant insects have had a technological edge against their equivalents. Their DDT-resistant properties have spread to more groups of the species, and now the mosquito community is governed by DDT-resistant. This is also an example of a natural selection. Correspondingly, as per the ‘Natural Selection concept’, insecticides could only remain effective for a short period. So, both assertion and reason are correct, and the reason is the correct explanation for the assertion.
Hence, the correct answer is option A.

Additional information: Synthetic pesticides are used for many ages to combat populations of insects and represent its most effective method of controlling pest-borne diseases, like mosquitoes. However, insects have battled against an evolving resistance to many chemicals. Now there are thousands of incidents of established resistance that cause certain chemical groups entirely inefficient.

Note: If several members of the population have such a resistance mutation, the chemical turns inefficient. When resistance evolves, it can spread to non-resistant mosquito vectors in other locations. Mosquitoes are highly mobile since they fly or are automatically carried at any time of their life cycle. Their movement would propagate mutations rapidly.