Select the correct statement:

Options

A. A. RNAi silencing takes place in all eukaryotic organisms as method of cellular defense.
B. B. RNAi requires silencing of mRNA by binding of complementary ssDNA molecule.
C. C. Complementary nucleic acid could be form from mobile genetic elements (transposons).
D. D. Ti plasmid with nematode-specific genes has been used in RNAi.

Answer

To solve the question of selecting the correct statement regarding RNA silencing and its mechanisms, we can analyze each of the provided statements step by step:

### Step-by-Step Solution:

1. **Evaluate the First Statement:**
- **Statement:** RNA silencing takes place in all eukaryotic organisms as a method of cellular defense.
- **Analysis:** This statement is correct. RNA silencing, also known as RNA interference (RNAi), is a process that occurs in eukaryotic cells to regulate gene expression and defend against viral infections by degrading double-stranded RNA (dsRNA). This prevents the production of proteins that could be harmful to the cell.

2. **Evaluate the Second Statement:**
- **Statement:** RNAi requires silencing of mRNA by binding of complementary single-stranded DNA molecule.
- **Analysis:** This statement is incorrect. RNAi involves the binding of complementary single-stranded RNA (not DNA) to mRNA, leading to its degradation. Therefore, the mention of single-stranded DNA is a misunderstanding of the process.

3. **Evaluate the Third Statement:**
- **Statement:** Complementary nucleic acid could be formed from mobile genetic elements which is known as transposon.
- **Analysis:** This statement is also incorrect. Transposons are segments of DNA that can move around within the genome, but they do not directly form complementary nucleic acids for RNA silencing. Instead, RNAi typically involves small interfering RNAs (siRNAs) that are derived from long double-stranded RNA.

4. **Evaluate the Fourth Statement:**
- **Statement:** Ti-plasmid with nematode specific gene has been used in RNAi.
- **Analysis:** This statement is incorrect. Ti-plasmids are used in the genetic engineering of plants, particularly for creating transgenic plants, but they are not specifically associated with RNAi mechanisms. Ti-plasmids do not contain nematode-specific genes for RNAi applications.

### Conclusion:
After evaluating all four statements, the correct answer is:
- **RNA silencing takes place in all eukaryotic organisms as a method of cellular defense.**