Transposons are

Options

A. Fragments of RNAi', that silences specific mRNA
B. ds RNA that initiates 'RNAi'
C. Mobile genetic elements that replicate via an RNA intermediate
D. Selectable markers like `'''"amp"''^R", "''tet^R''"` etc

Answer

### Step-by-Step Solution:

1. **Understanding Transposons**: Transposons, also known as "jumping genes," are segments of DNA that can move around within the genome. They can change their position, which can lead to mutations and alterations in the genetic material.

2. **Identifying the Options**: The question provides four options regarding what transposons are:
- A) Fragments of RNA that silence specific mRNA.
- B) Double-stranded RNA that initiates RNA interference.
- C) Mobile genetic elements that replicate via an RNA intermediate.
- D) Selectable markers like ampicillin resistance and tetracycline resistance.

3. **Evaluating Each Option**:
- **Option A**: This describes micro RNA, which is not a transposon. **(Incorrect)**
- **Option B**: This refers to small interfering RNA (siRNA), which also does not describe transposons. **(Incorrect)**
- **Option C**: This option correctly describes transposons as mobile genetic elements that can replicate and move within the genome. **(Correct)**
- **Option D**: This option describes selectable markers used in genetic engineering, which are not transposons. **(Incorrect)**

4. **Conclusion**: The correct answer is **C) Mobile genetic elements that replicate via an RNA intermediate**. Transposons are indeed mobile genetic elements that can change their position within the genome.