Which organism converts human excreta into vermi -cakes in vermi processing toilets? 

Earthworms 

The correct answer is option 2. Earthworms.

Out of the listed options, earthworms are the primary organisms responsible for converting human excreta into vermi-cakes in vermi processing toilets.

Here's why:

Bacteria and microorganisms are certainly present in vermi processing toilets and play a role in the decomposition process, breaking down organic matter in the excreta. However, they are not the primary drivers of the conversion into vermi-cakes.

Earthworms, particularly red wiggler worms (Eisenia foetida), are specifically chosen for vermi processing toilets. They ingest the human excreta and other organic matter, break it down in their digestive system, and then excrete it as castings, which are also known as vermi-cakes. These vermi-cakes are a valuable nutrient-rich fertilizer enriched by the earthworms' gut flora.

Therefore, while bacteria and microorganisms contribute to the overall process, earthworms are the key players in converting human excreta into vermi-cakes in vermi processing toilets.