Bacillus thuringiensis is used to control :
(a)Bacteria pathogens
(b)Fungal pathogens
(c)Nematodes
(d)Insect pests

Hint: Bacillus thuringiensis is a bacteria that is used as a biocontrol agent in agriculture as they can release a lethal protein specific to a particular set of organisms. Biocontrol refers to the biological methods for controlling plant diseases.

Complete answer:
Bacillus thuringiensis is the most widely applied species of bacteria used as a biocontrol agent with as many as three subspecies to control insect pests such as Lepidopteran (moth, butterfly), Coleopteran( beetle) and Dipteran (true flies). They produce protein crystals during a particular phase of their growth. These crystals contain a toxic insecticidal protein.

Additional Information: Bacillus thuringiensis (Bt) toxic protein exists as an inactive protoxin in intracellular crystalline bodies. Once they are ingested by an insect, the protoxin is converted into an active toxin due to the solubilization of crystals in the alkaline pH of the insect’s gut. The toxin binds to the surface of the insect’s midgut epithelial cells and perforates it. Formation of pores causes cell swelling, lysis and eventually death of the insect
Through genetic engineering, it is possible to develop crops that are intrinsically resistant to insects. The gene of Bacillus thuringiensis which codes for this insecticidal toxic protein has been introduced in various crop plants so that if any insect attempts to eat it, they would be killed. Bt toxins can be applied to the crops in the form of spray also. In 1995, the Environmental Protection Agency approved the release of genetically engineered Bt crops such as cotton, potatoes, corn, and rice.
So, the correct answer is ‘Insect pests’.

Note: One of the advantages of Bt toxin is its narrow range of the spectrum. As many other insecticides kill various beneficial insects which may be pollinators such as bees, Bt only targets an individual insect species. Also, genetically engineered insect-resistant varieties of plants have reduced the dependency on chemical insecticides which are poisonous as well as expensive.