Which of the following is considered a microbial pesticide?

The correct choice is that boric acid is designated as a microbial pesticide due to its activity against various fungi, particularly those that can damage plants. Microbial pesticides are defined as products containing microorganisms, such as bacteria, fungi, viruses, or protozoa, which can control pests or diseases.

In this context, while pyrethrum is derived from chrysanthemum flowers and acts as a botanical insecticide, it is not classified as a microbial pesticide. Basic copper sulfate is a copper-based fungicide often used for its ability to control fungal diseases and is classified as a chemical pesticide instead. Lime, primarily used to adjust soil pH, does not fit the definition of a pesticide at all; rather, it contributes to soil health and plant nutrition.

Therefore, understanding that microbial pesticides specifically involve living organisms or their byproducts which target pest populations clarifies why boric acid, used effectively for controlling certain microbial pests, fits within this category, making it the appropriate answer.