What does "Because of increasing resistance of mosquitos to" modify?
I say increase. But what about what modifies "incidence of malaria".?
Take out the "because of" so you can just look at the subject. Resistance
is the subject of the clause. It is not correct to say "resistance against" - it's redundant. That's the biggest reason why (A) is wrong here. "Resistance to" is the proper term.