In a certain area, mosquitoes that can resist the effects of pesticides (insect-killing chemicals) now exist in greater numbers than10 years ago. What is the most probable explanation for this increase in numbers?

They have developed a resistance to the active killing ingredient of the pesticide. Their immune system has figured out how to deal with the active ingredient. Apparently it is an example of survival of the fittest. They are able to pass that quality along to their offspring.