Mutation is a phenomenon which cells results in alteration of DNA sequence.

Answer

### Step-by-Step Text Solution:

1. **Definition of Mutation**:
- Mutation refers to a sudden change in the DNA sequence of an organism. This alteration can occur in various forms, affecting the genetic information carried by the DNA.

2. **Causes of Mutation**:
- Mutations can be caused by several factors:
- **Chemical Agents**: Certain chemicals can interact with DNA and cause changes in the sequence.
- **Errors in DNA Replication**: During the process of DNA replication, mistakes can occur, leading to mutations.
- **Radiation**: Exposure to various types of radiation, such as X-rays and gamma rays, can also induce mutations in the DNA.

3. **Effect of Mutation**:
- The result of a mutation is an alteration in the DNA sequence, which can lead to changes in the organism's traits or functions. This alteration is what defines a mutation.

4. **Conclusion**:
- Therefore, the statement that "mutation is a phenomenon which results in alteration of DNA sequence" is true. Mutations represent significant changes in the genetic code that can have various biological implications.