Dragonflies: Mosquitoes :: Ladybird beetles : ________

The correct option is A Aphids
Biocontrol is the method of controlling pests that relies on natural predation rather than introduced chemicals. It greatly reduces our dependence on toxic chemicals and pesticides. Example -

1. Dragonflies are used to get rid of mosquitoes - Dragonflies are natural predators for mosquitoes. A single dragonfly can eat about 30 to hundreds of mosquitoes per day and thus provide a great control on the mosquito population.
2. Ladybird beetles are useful to get rid of aphids - They feed on aphids voraciously. Mature ladybugs will feed on 20 to 25 aphids per day while their larvae are more effective predators and consume about 10 times more than the adults.

The bacterium Bacillus thuringiensis (Bt) is a microbial biocontrol agent used to control butterfly caterpillars. These are available in sachets as dried spores which are mixed with water and sprayed onto vulnerable plants such as brassicas and fruit trees, where these are eaten by the insect larvae. In the gut of the larvae, the toxin is released and the larvae get killed. The bacterial disease will kill the caterpillars, but leave other insects unharmed.
Nucleopolyhedrovirus is a genus belonging to baculoviruses. They are pathogens of insects and other arthropods. They are used as biological control agents for species-specific, narrow spectrum insecticidal applications.