Which of the following is a naturally occurring fungus formulated as an insecticide?

Beauveria bassiana is a naturally occurring fungus that has been recognized for its insecticidal properties. It acts as a biocontrol agent, infecting and killing various insect pests, particularly those in the orders Coleoptera (beetles) and Lepidoptera (moths and butterflies). Once the spores of Beauveria bassiana come into contact with the insect, they germinate and penetrate the insect's exoskeleton, leading to the insect's death by causing a variety of physiological disruptions. This makes it an effective choice for pest management in organic and sustainable agriculture.

The other options, while being various species of fungi, do not primarily serve as insecticides. Botrytis cinerea is known for causing gray mold in plants, Fusarium oxysporum is associated with plant diseases and root rot, and Pythium arrhenomanes generally causes damping-off disease in seedlings. None of these fungi have the same insecticidal functionality as Beauveria bassiana.