Bacillus thuringiensis is a biocontrol agent against :

The correct answer is Option (3) → Insect pests

Biocontrol is the use of biological methods to control plant diseases and pests.

Option (1) Nematode — Incorrect. Nematodes are controlled by RNA interference in plants, not by Bt.

Option (2) Fungal pathogen — Incorrect. The biocontrol agent for plant pathogens named in the book is the fungus Trichoderma.

Option (3) Insect pests — Correct. Bacillus thuringiensis spores are sprayed on plants. When insect larvae such as butterfly caterpillars eat them, the toxin is released in their gut and kills them.

Option (4) Bacterial pathogen — Incorrect. Bt is itself a bacterium; it is not used against bacteria.

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 ). 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.”