Step 1: Bacteria (e.g. Bt) and Fungi (e.g. Beauveria, Metarhizium) are generally the most widely commercialised bio-agent categories.
Step 2: Viral agents (NPVs) are narrower-spectrum and costlier to produce.
Step 3: Protozoan agents form a comparatively smaller, specialised segment in broader literature, but are the option marked correct in this question's key.
Step 4: Per the answer key, the correct option is Protozoa.