Amoeba is called immortal because it does not undergo natural death.

Answer

### Step-by-Step Text Solution:

1. **Understanding the Concept of Immortality in Amoeba**:
- The term "immortal" in the context of organisms like amoeba refers to their ability to avoid natural death. Unlike many multicellular organisms that have a defined lifespan, amoeba does not exhibit signs of aging or natural death.

2. **Lack of Natural Death**:
- Amoeba does not undergo natural death, which means that it does not die from age-related causes or other natural factors. This characteristic is what leads to the description of amoeba as "immortal."

3. **Reproduction Process**:
- Amoeba reproduces asexually through a process called binary fission. In this process, the parent amoeba divides into two identical daughter cells. Each daughter cell can then grow and divide, continuing the cycle of life.

4. **Absence of Parent Body Remains**:
- To date, there have been no remains of a parent amoeba found that indicate it has died naturally. This further supports the idea that amoeba does not experience natural death.

5. **Conclusion**:
- Therefore, the statement "Amoeba is called immortal because it does not undergo natural death" is true. Amoeba's unique reproductive method and lack of natural death contribute to its classification as an immortal organism.