_________a crown gall bacterium, is called an natural genetic engineer' of plants.
1) Escherichia coli
2) Streptomyces albus
3) Agrobacterium tumefaciens
4) Azotobacter

Options

A. 1) Escherichia coli
B. 2) Streptomyces albus
C. 3) Agrobacterium tumefaciens
D. 4) Azotobacter

Explanation

To answer the question, "_________ a crown gall bacterium, is called a natural genetic engineer of plants," we need to identify which of the provided options is the correct name of the crown gall bacterium.

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the name of a bacterium that is known to cause crown gall disease in plants and is referred to as a "natural genetic engineer" of plants.

2. **Analyzing the Options**:
- **Option 1: Escherichia coli** - This bacterium is primarily found in the intestines of humans and is not known to cause diseases in plants. Therefore, it cannot be the answer.
- **Option 2: Streptomyces albus** - This is a type of bacterium that is not a plant pathogen and does not cause crown gall disease. Thus, it is also not the answer.
- **Option 3: Agrobacterium tumefaciens** - This bacterium is well-known for causing crown gall disease in plants. It is often referred to as a "natural genetic engineer" because it transfers DNA to plant cells, leading to tumor formation.
- **Option 4: Azotobacter** - This bacterium is involved in nitrogen fixation and does not cause diseases in plants, making this option incorrect as well.

3. **Selecting the Correct Answer**: After analyzing all the options, it is clear that **Agrobacterium tumefaciens** is the correct answer as it is the bacterium responsible for crown gall disease and is recognized for its ability to transfer genetic material to plants.

### Final Answer:
**Agrobacterium tumefaciens**

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