Which of the following statement(s) is/are correct with regard to bio…

The correct answer is Option (4) → All of the above statements are correct.

Biocontrol is the use of living organisms, instead of chemicals, to control plant diseases and pests. Predators, bacteria, fungi and viruses can all be biocontrol agents.

Option (1) Ladybird beetles are employed to eliminate aphids, while dragonflies are used to control mosquitoes. — Correct statement, but it is only one of the correct points, so on its own it is not the answer. The ladybird beetle (red with black spots) eats aphids, and dragonflies eat mosquitoes. Both are useful predators.

Option (2) Bacillus thuringiensis bacteria are used to control butterfly caterpillars. — Correct statement, but it is only one of the correct points, so on its own it is not the answer. Spores of Bacillus thuringiensis (Bt) are sprayed on plants. The caterpillars eat them, a toxin is released in their gut, and the larvae are killed.

Option (3) Trichoderma species are employed for the management of various plant pathogens. — Correct statement, but it is only one of the correct points, so on its own it is not the answer. Trichoderma species are free-living fungi common around roots. They are effective biocontrol agents of several plant pathogens.

NCERT Extract:
Microbes as Biocontrol Agents – “… The very familiar beetle with red and black markings – the Ladybird, and Dragonflies are useful to get rid of aphids and mosquitoes, respectively. An example of microbial biocontrol agents that can be introduced in order to control butterfly caterpillars is the bacteria Bacillus thuringiensis (often written as Bt). … A biological control being developed for use in the treatment of plant disease is the fungus Trichoderma. Trichoderma species are free-living fungi that are very common in the root ecosystems. They are effective biocontrol agents of several plant pathogens.”