One of the methods of which DNA cannot be transferred to the host cell is by

Options

A. Microinjection
B. Gene gun
C. Disarmed pathogen vectors
D. PCR

Answer

To solve the question "One of the methods of which DNA cannot be transferred to the host cell is by," we will analyze each of the options provided.

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for a method that does not facilitate the transfer of DNA into a host cell.

2. **Analyzing the Options**:
- **Microinjection**: This is a technique where DNA is directly injected into the nucleus of a cell using a fine glass needle. This method is effective for transferring DNA into host cells. Therefore, this option is **not** the correct answer.

- **Gene Gun**: This method, also known as biolistics, involves shooting microscopic gold or tungsten particles coated with DNA into cells. This technique is also effective for transferring DNA into host cells. Hence, this option is **not** the correct answer.

- **Disarmed Pathogen Vectors**: This refers to using modified viruses or bacteria that can deliver DNA into host cells without causing disease. This method is commonly used in genetic engineering and is effective for DNA transfer. Thus, this option is **not** the correct answer.

- **PCR (Polymerase Chain Reaction)**: PCR is a technique used to amplify DNA, meaning it increases the quantity of DNA but does not transfer it into host cells. Therefore, this method does not facilitate the transfer of DNA into host cells. This option is the correct answer.

3. **Conclusion**: After analyzing all the options, the method by which DNA cannot be transferred to the host cell is **PCR**.

### Final Answer:
The correct answer is **PCR (Polymerase Chain Reaction)**.