Microbial biocontrol agent for butterfly caterpillar is : 

Options

A. Bacillus thuringierisis
B. Saccharomyces
C. Lactobacillus
D. Cyanobacteria

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a microbial biocontrol agent specifically used to control butterfly caterpillars. This indicates that we need to identify a microorganism that can effectively manage or eliminate this pest.

2. **Identifying the Problem**: Butterfly caterpillars are pests that can damage crops and plants. Therefore, controlling their population is crucial for agricultural health.

3. **Exploring Biocontrol Agents**: Biocontrol agents are organisms that can naturally control pest populations. They can include bacteria, fungi, and other microorganisms.

4. **Finding the Right Microbial Agent**: The specific microbial agent known for controlling butterfly caterpillars is **Bacillus thuringiensis** (often abbreviated as Bt). This bacterium produces a toxin that is harmful to certain insects.

5. **Mechanism of Action**: The toxin produced by Bacillus thuringiensis is inactive when ingested. However, when it reaches the alkaline environment of the insect's midgut, it becomes activated. The activated toxin then damages the cells of the caterpillar's gut, leading to swelling and eventual death of the insect.

6. **Evaluating the Options**:
- Option 1: Bacillus eugenesis (incorrect)
- Option 2: Saccharomyces (yeast, used for fermentation, incorrect)
- Option 3: Lactobacillus (used in fermentation, incorrect)
- Option 4: Cyanobacteria (not a biocontrol agent for caterpillars, incorrect)

7. **Conclusion**: Based on the information, the correct answer is **Bacillus thuringiensis** as it is the recognized microbial biocontrol agent for butterfly caterpillars.

**Final Answer**: Bacillus thuringiensis

---